I might be missing something, but I think the aarch64 case can still leak.
In configure.ac, right after the cached sysconf check there's:

  AS_CASE([$build_cpu], [aarch64*], ... erofs_cv_max_block_size=16384 ...)

so on an aarch64 worker building qemux86-64 the cached 4096 still gets bumped to 16384.
When I tried the v1.9.4 configure with --build=aarch64-linux --host=x86_64-linux, config.h ended up with 16384.

Passing MAX_BLOCK_SIZE=${BLOCKSIZE} instead seems to skip that whole block.
The next 119 run should show it, as long as reproducibleA picks up erofs-utils from x86_64 sstate.

Thanks,
Sunil