With showSignature option added for git log,

$ cat ~/.gitconfig
# This is Git's per-user configuration file.
[log]
	showSignature = true

uscan fails to create the orig.tar with correct name in git mode

version=4
opts="mode=git,pgpmode=none" \
https://github.com/blakeembrey/change-case.git HEAD

$ uscan --verbose
uscan info: uscan (version 2.21.3) See uscan(1) for help
uscan info: Scan watch files in .
find: File system loop detected; './node_modules/pascal-case' is part
of the same file system loop as '.'.
uscan info: Check debian/watch and debian/changelog in .
uscan info: package="node-pascal-case" version="3.1.2-1" (as seen in
debian/changelog)
uscan info: package="node-pascal-case" version="3.1.2" (no
epoch/revision)
uscan info: ./debian/changelog sets package="node-pascal-case"
version="3.1.2"
uscan info: Process watch file at: debian/watch
    package = node-pascal-case
    version = 3.1.2
    pkg_dir = .
uscan info: opts: mode=git,pgpmode=none
uscan info: line: https://github.com/blakeembrey/change-case.git HEAD
uscan info: Parsing mode=git
uscan info: Parsing pgpmode=none
uscan info: line: https://github.com/blakeembrey/change-case.git HEAD
uscan info: Last orig.tar.* tarball version (from debian/changelog):
3.1.2
uscan info: Last orig.tar.* tarball version (dversionmangled): 3.1.2
uscan info: Looking at $base =
https://github.com/blakeembrey/change-case.git with
    $filepattern = HEAD found
    $newfile = HEAD
    $newversion = gpg: Warning: using insecure memory!
gpg: Signature made Mon May 10 00:19:00 2021 UTC
gpg: using RSA key 4AEE18F83AFDEB23
gpg: Can't check signature: No public key
0.0~git20210510.c24a0e1
    $lastversion = 3.1.2
uscan info: Upstream URL(+tag) to download is identified as
https://github.com/blakeembrey/change-case.git HEAD
uscan info: Filename (filenamemangled) for downloaded file:
node-pascal-case-gpg: Warning: using insecure memory!
gpg: Signature made Mon May 10 00:19:00 2021 UTC
gpg: using RSA key 4AEE18F83AFDEB23
gpg: Can't check signature: No public key
0.0~git20210510.c24a0e1.tar.xz
uscan info: Newest version of node-pascal-case on remote site is gpg:
Warning: using insecure memory!
gpg: Signature made Mon May 10 00:19:00 2021 UTC
gpg: using RSA key 4AEE18F83AFDEB23
gpg: Can't check signature: No public key
0.0~git20210510.c24a0e1, local version is 3.1.2
uscan info: => Only older package available from:
             => https://github.com/blakeembrey/change-case.git HEAD
uscan info: Removing git repo (../node-pascal-case-temporary.384.git)
uscan info: Scan finished

The filenames have the error messaages in them

$ ls ..
 node-pascal-case-3.1.2
'node-pascal-case-gpg: Warning: using insecure memory!'$'\n''gpg:
Signature made Mon May 10 00:19:00 2021 UTC'$'\n''gpg: using RSA key
4AEE18F83AFDEB23'$'\n''gpg: Can'\''t check signature: No public
key'$'\n''0.0~git20210510.c24a0e1.tar.xz'
 node-pascal-case_3.1.2-1.debian.tar.xz
 node-pascal-case_3.1.2-1.dsc
 node-pascal-case_3.1.2.orig.tar.gz
'node-pascal-case_gpg: Warning: using insecure memory!'$'\n''gpg:
Signature made Mon May 10 00:19:00 2021 UTC'$'\n''gpg: using RSA key
4AEE18F83AFDEB23'$'\n''gpg: Can'\''t check signature: No public
key'$'\n''0.0~git20210510.c24a0e1.orig.tar.xz'
 node-pascal-case_itp.mail
 pascal-case-3.1.2.tgz

after commenting out the line in .gitconfig, it works

$ uscan --verbose --force-download
uscan info: uscan (version 2.21.3) See uscan(1) for help
uscan info: Scan watch files in .
find: File system loop detected; './node_modules/pascal-case' is part
of the same file system loop as '.'.
uscan info: Check debian/watch and debian/changelog in .
uscan info: package="node-pascal-case" version="3.1.2-1" (as seen in
debian/changelog)
uscan info: package="node-pascal-case" version="3.1.2" (no
epoch/revision)
uscan info: ./debian/changelog sets package="node-pascal-case"
version="3.1.2"
uscan info: Process watch file at: debian/watch
    package = node-pascal-case
    version = 3.1.2
    pkg_dir = .
uscan info: opts: mode=git,pgpmode=none
uscan info: line: https://github.com/blakeembrey/change-case.git HEAD
uscan info: Parsing mode=git
uscan info: Parsing pgpmode=none
uscan info: line: https://github.com/blakeembrey/change-case.git HEAD
uscan info: Last orig.tar.* tarball version (from debian/changelog):
3.1.2
uscan info: Last orig.tar.* tarball version (dversionmangled): 3.1.2
uscan info: Looking at $base =
https://github.com/blakeembrey/change-case.git with
    $filepattern = HEAD found
    $newfile = HEAD
    $newversion = 0.0~git20210510.c24a0e1
    $lastversion = 3.1.2
uscan info: Upstream URL(+tag) to download is identified as
https://github.com/blakeembrey/change-case.git HEAD
uscan info: Filename (filenamemangled) for downloaded file:
node-pascal-case-0.0~git20210510.c24a0e1.tar.xz
uscan info: Newest version of node-pascal-case on remote site is
0.0~git20210510.c24a0e1, local version is 3.1.2
uscan info: => Only older package available from:
             => https://github.com/blakeembrey/change-case.git HEAD
uscan info: => Forcing download as requested
uscan info: Downloading upstream package:
node-pascal-case-0.0~git20210510.c24a0e1.tar.xz
uscan info: Successfully downloaded package:
node-pascal-case-0.0~git20210510.c24a0e1.tar.xz
uscan info: Missing OpenPGP signature.
uscan info: New orig.tar.* tarball version (oversionmangled):
0.0~git20210510.c24a0e1
uscan info: Launch mk-origtargz with options:
   --package node-pascal-case --version 0.0~git20210510.c24a0e1 
--compression default --directory .. --copyright-file debian/copyright 
./node-pascal-case-0.0~git20210510.c24a0e1.tar.xz
Successfully symlinked
./node-pascal-case-0.0~git20210510.c24a0e1.tar.xz to
./node-pascal-case_0.0~git20210510.c24a0e1.orig.tar.xz.
uscan info: New orig.tar.* tarball version (after mk-origtargz):
0.0~git20210510.c24a0e1
uscan info: Removing git repo (../node-pascal-case-temporary.508.git)
uscan info: Scan finished

$ ls ..
node-pascal-case-0.0~git20210510.c24a0e1.tar.xz
node-pascal-case-3.1.2
node-pascal-case_0.0~git20210510.c24a0e1.orig.tar.xz
node-pascal-case_3.1.2-1.dsc
node-pascal-case_itp.mail
pascal-case-3.1.2.tgz