The FreeBSD em driver fails to properly reset the VME flag in the e1000 CTRL register during the following ifconfig command
ifconfig em1 -vlanhwtag
at least on the e1000 device as emulated by QEMU
https://github.com/luigirizzo/netmap/issues/703
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236584