Comment 7 for bug 2061667

And your second reproducer, with /mp in auto.master, doesn't work here:

# ls -la /mp/mumon/_.
ls: cannot access '/mp/mumon/_.': No such file or directory

handle_packet: type = 3
handle_packet_missing_indirect: token 43, name mumon, request pid 4242
attempting to mount entry /mp/mumon
lookup_mount: lookup(file): looking up mumon
key "mumon" not found in map source(s).
dev_ioctl_send_fail: token = 43
failed to mount /mp/mumon