Thanks, I put everything in place... I am now waiting for the next
occurrence...

The best way to debug such problem is to recompile the kernel adding
some debug outputs.
(Maybe it can be done with bpftrace, but not yet familiar with that)

If you're able to recompile the kenerl (using abs + makepkg for Arch
based kernel), please try the following diff.

This will add extra debugging to show where the offending length
happens, either extent discard or unallocated space discard.
And from that output we can continue our investigation.

Thanks,
Qu