Today I observed a new class of issues that affects at least
adinatha-fonts arabic-fonts gdouros-akkadian-fonts icingaweb2 solaar xsane

I did not re-test all packages.
There are unaffected packages.
Affected package sizes range from 1MB to 5MB.

I made a minimal reproducer in
https://build.opensuse.org/package/show/home:bmwiedemann:reproducible:test/adinatha-fonts

Building it with 
for N in 1 2 ; do
 osc build --noservice --clean --vm-type=kvm -j$N \
 --keep-pkgs=pkgs.$N openSUSE_Tumbleweed x86_64 ; done
causes it to vary

It probably has to do with

filterdiff printrpmtags pkgs.[12]/adinatha-fonts-1.0-0.noarch.rpm
 LONGARCHIVESIZE=2146852
 LONGFILESIZES=0
-LONGSIGSIZE=1897928
+LONGSIGSIZE=1897768
 LONGSIZE=2146214


What recent change could cause this? Does it have to do with compression?
There have been previous cases where single-threaded compression gave different
results, but afaik zstd avoided these by always using the multi-threaded code,
even on single-core.