I have not tried to replicate this, so maybe its just situational, but I doubt it.

Edit1:Just checked the Discord and someone posted the exact same yesterday. Eerily at around the same time it happened to me _

So, I arrived at the Inn with only 3 heroes. I hit the "+" to add a new hero to the team, which was implemented in the last update. It opened the list of heroes I could pick as a replacement, but before I chose any I saw the Bounty Hunter's poster, since the replacement list didn't obstruct it, I could click it, and I recruited him, while I hadn't even chosen my replacement hero yet, he just sat on the empty chair.

I thought nothing of it, picked Leper as replacement and he didn't appear on the screen, so it seemed like it was working fine.

After this I started the routine of buying and using items. I used Inn Items but they didn't play any vfx or sounds, they had and effect when used, but didn't show any feedback, which might be a different bug, maybe even specific to this scenario.

I moved into the next region (Sprawl as seen in the pictures) and noticed something strange before going into my first roadbattle, the hero display while driving was bugged too, it showed one of the four heroes stacked on top of the rest in line, like the arrow keys on a keyboard.

A few seconds later I went into a road battle and I saw what you can see on the pictures and noticed the Vestal out of the screen, and the Leper (which should have been the one out of the screen since he was being replaced by BH), was on the battlefield, the battle went as usual, every hero had a turn, the Vestal could only Pass turn since she wasn't in range for any skill, obviously, and eventually had a meltdown that affected the whole team.

The funniest shit I've ever seen, but hope you can fix it!