NFS exports seem to die after five minutes

For years now, I have exported an NFS share from a host Mac which I connect to from a Raspberry Pi on the same network. I configure this by adding a line in /etc/exports - /Users/Pi -mapall=myusername

This has always worked flawlessly, but since updating my Mac (M4 Mac Mini) to Sequoia 15.5 last week, it has developed a problem. If the NFS share is not accessed from the Pi for five minutes, it dies, and the Pi's file manager locks up necessitating a complete reboot.

If I run a script on the PI which does an ls on the mounted share every 5 minutes, the lockup does not happen. But if I extend the period to 6 minutes, the lockup occurs.

Something on the Mac NFS server seems to be dying in an unrecoverable fashion after five minutes of idle. Even with nfsd logging set to verbose, there is nothing helpful in the console logs.

I am open to suggestions to further investigate or to try and fix this, but this is basically a showstopper for me - I need to be able to share data between Mac and Pi, and this is now broken.