I don't think these are the same as previously encountered native-compilation related failures. I get the same / similar failures when running EMACS_INHIBIT_AUTOMATIC_NATIVE_COMPILATION=t emacs -q -batch -L . -l buttercup -f buttercup-run-discover as a non-root user with a defined home directory. Log is attached (there is one more failure in the log, iiuc related to gpg missing in the chroot). I tried with the upstream master branch, which contains an upstream change related to buttercup and emacs 28, and I still get the same 4 failures, including the two buffer-file-name related failures.
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buttercup.log
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