I actually tried a FreeBSD 13 image on the rpi4 and got the same not showing up on the network or replying to pings issue. I think that FreeBSD 13 on rpi4 might just not be ready yet. I wish I had a micro-HDMI cable to diagnose things further.
I think someone reported the latest firmware or something pulling some shenanigans if there's no display connected, whether we use it or not. I don't recall the specifics at the moment, though.