Created attachment 712371 [details] output of 'rc-service -d nfs start' 'rc-service nfs start' seemingly starts without problems but remote hosts are not able to mount the exported filesystems. The same config wokrs just fine when I run the system from the systemd/glibc partition (even when some hosts are not known).
Created attachment 712374 [details] /etc/exports
Created attachment 712377 [details] emerge --info
You seem to be using nfs-utils::gentoo , which as of 2.5.3 compiles fine, though I don't know if it works fine. Alpine are carrying 3 patches for 2.5.3, so I assume those are still required. I've bumped nfs-utils in ::musl to 2.5.3 with those patches. Can you test?
Recently tried again with net-fs/nfs-utils-2.5.4-r4::gentoo and now everything worked as expected out of the box. Can't reproduce the issue any longer, so closing here.