The debugging symbols have been embedding build paths since 2022.04~rc4.
Not sure if it was a packaging change, or an upstream change, but passing -ffile-prefix-map instead of -fmacro-prefix-map to the compiler, as well as passing --debug-prefix-map to the "as" generated assembly fixed it!
Time to submit upstream!
What made it exceptionally confusing was my attempts to trigger the issue locally failed. For some reason the debugging symbols were not being used in my local builds...
I tried cross-building the failing targets as an arch:all package, but then there were no debugging symbols, as they were manually stripped out; dh_strip doesn't strip arch:all... I think... and so the dbgsym packages were not created.
So I removed the manual stripping and I could reproduce the reproducibility issue!
For people who care about, support, or build Free, Libre, and Open Source Software (FLOSS).