Yeah that's another issue we noted recently where bsdinstall never populated the ESP on additional members of a mirror and such.
There is a review about it upstream as well: https://reviews.freebsd.org/D42150
Though I don't think I've ever seen a BIOS randomly decide to boot from the second disk like that, usually people don't notice the problem until the first disk of a mirror fails in some way and then it won't boot even when the second disk was a good mirror of the first otherwise.