The RPATH contains the build path resulting in different buildid:

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/mathgl.html

The attached patch to debian/rules passes
-DCMAKE_BUILD_RPATH_USE_ORIGIN=ON via a dh_auto_configure override,
which should use a relative path for RPATH.

Alternately, updating the packaging to debhelper compat level 14 should
fix this, although it is currently an experimental compat level.

With this patch applied, mathgl should build reproducibly on
tests.reproducible-builds.org!

Thanks for maintaining mathgl!

live well,
  vagrant

We believe that the bug you reported is fixed in the latest version of
mathgl, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1003915@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Rafael Laboissière <rafael@debian.org> (supplier of updated mathgl package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Mon, 30 May 2022 07:04:37 -0300
Source: mathgl
Architecture: source
Version: 8.0.1-2
Distribution: unstable
Urgency: medium
Maintainer: Debian Science Maintainers <debian-science-maintainers@lists.alioth.debian.org>
Changed-By: Rafael Laboissière <rafael@debian.org>
Closes: 1003915
Changes:
 mathgl (8.0.1-2) unstable; urgency=medium
 .
   [ Rafael Laboissière ]
   * d/control: Bump Standards-Version to 4.6.1 (no changes needed)
 .
   [ Vagrant Cascadian ]
   * d/rules: Pass -DCMAKE_BUILD_RPATH_USE_ORIGIN=ON via dh_auto_configure override
     (Closes: #1003915)
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