kern.proc.pathname failure while booting from zfs

Hi

I see a strange interaction between zfs on root and kern.proc.pathname
on my laptop. Whenever I try to use gcore it fails with:
gcore 1023
gcore: kern.proc.pathname failure

However, gcore /usr/local/bin/zsh 1023 is working properly.

I made some tests booting from usb stick (fresh installworld, no
src.conf, no make.conf, GENERIC kernel)
What works: having / on ufs and importing a zfs pool later on.
What doesn't: having / on zfs, whatever the settings for checksum,
compression, or normalization.

Both 11-stable and 12-current behave this way. Current from may-june
worked properly.
adb, chromium and virtualbox as well stopped working at approximately
the same time, however I don't know if it is linked ("truss -f adb
start-server" shows that garbage is passed to execl after forking).

Any idea what's going on? Does anybody else see this?

Thanks!