With no explicit version it gets 0.12~pre6-5:

$ apt-get source elinks
Reading package lists... Done
Building dependency tree
Reading state information... Done
Skipping already downloaded file 'elinks_0.12~pre6-5.dsc'
Skipping already downloaded file 'elinks_0.12~pre6.orig.tar.gz'
Skipping already downloaded file 'elinks_0.12~pre6-5.debian.tar.xz'
Need to get 0 B of source archives.
Skipping unpack of already unpacked source in elinks-0.12~pre6

However, it knows about and is capable of fetching 0.12~pre6-8:

$ apt-get source elinks=0.12~pre6-8
Reading package lists... Done
Building dependency tree
Reading state information... Done
Skipping already downloaded file 'elinks_0.12~pre6-8.dsc'
Skipping already downloaded file 'elinks_0.12~pre6.orig.tar.gz'
Skipping already downloaded file 'elinks_0.12~pre6-8.debian.tar.xz'
Need to get 0 B of source archives.
Skipping unpack of already unpacked source in elinks-0.12~pre6

$ apt-cache policy
Package files:
 100 /var/lib/dpkg/status
     release a=now
 500 http://ftp.debian.org/debian/ unstable/main Translation-en
 500 http://ftp.debian.org/debian/ unstable/main arm64 Packages
     release o=Debian,a=unstable,n=sid,l=Debian,c=main
     origin ftp.debian.org
Pinned packages:

This is probably related somehow to the fact that
http://ftp.debian.org/debian/dists/unstable/main/source/Sources.xz
currently contains sections for both

    Package: elinks
    Version: 0.12~pre6-5

and

    Package: elinks
    Version: 0.12~pre6-8

in that order. (Perhaps it's keeping 0.12~pre6-5 because that's the
last version that was successfully built on arm64, but the man page
for apt-get just says "newest available version", not "newest
available version that was successfully built on this arch".)