Based on everything I know about this game, Regenerate-and-run should count as 2 turns, first, the turn of the Regeneration (during which the Vulnerability should wear off) and second, the turn of the swap, which includes the SIA.
Here,the Vulnerability lasts for both the turns, buffing Dracoceratops’ damage, when it shouldn’t.
If this isn’t a bug, then that would mean that a lot of other previous complaints from other users are bugs. So I’m pretty confident it’s a bug.
I’m pretty sure that when I face Paramoloch in the arena it is counted as a turn, the same way Regeneration is counted as a turn while facing Miragaia.
This is probably just a problem with Vulnerability or Bajatonodon.
I think I know where is the problem. Bajatonodon is faster. Description of Superior vulnerability says it lasts until the end of the next turn. So Bajatonodon uses Regenerate & Run to DC before opponent can make a move. Opponents move is needed to end turn. If Bajatonodon would be slower, vulnerability would end before DC enters field.