justbuild currently hard-codes use of clang-19 from llvm-toolchain-19
(which has several RC bugs and is likely to be removed from testing).
Comments in debian/justbuild.makefile suggest that this is because the
current version of fmtlib in unstable only works with older clang:

By default, Debian builds most packages with gcc, and I see that
justbuild is compiled with gcc on riscv64. Would it be possible to
build justbuild with gcc on all architectures? That would mean it's no
longer blocked by #1114025, and no longer in danger of autoremoval.

Thanks,
    smcv