The module is not auto-loaded on sparc64 when a request is made to perform an nfsd system call. The bug appears to be in the sparc64 32-bit syscall compat layer. I will work on a fix and see why it is not performing the module load via kmod as it should.
Seems to be working in 5.9.8 build, perhaps it was a module conf problem or similar. I will re-open this if I see it triggered again.