ZFS system can no longer boot - zfs_alloc()/zfs_free() mismatch

[Joseph Mingrone]

Hello everyone,

As it says in the subject line, my system stopped booting for some
unknown reason.  It might have been after upgrading from 12.1-p2 to
-p4, but I'm unsure.  It sounds similar to this issue:
https://twitter.com/alelab_/status/1222437570501644291

Allan and I tried debugging for a few hours today, but without
success.  We tried replacing the boot code and the loaders, but the
result was the same.  We also tried booting from a usb thumb drive,
then `set currdev=zfs:zroot/ROOT/default:` and `boot`, but the same
error is shown.

Steve and Kyle, did you ever solve this problem?

Thanks,

Joe