Shutdown causes a pool lock

I recently added an Optane SLOG and since when I have shutdown, using

shutdown -p now

and in each case on reboot the pool is locked:

LOCKED    lock This geli-encrypted pool failed to decrypt. 

After recovering by removing the SLOG and adding it back all is well.

Does anyone have an explanation, please?

It could be an edge case related to GELI encryption, have you considered migrating to a ZFS-encrypted pool?

Storage Encryption | TrueNAS Documentation Hub.

Yes, it did occur to me, but I was procrastinating, hoping the issue lay elsewhere.
Thanks for the nudge,