I feel like it’s not something that was guaranteed to be found even if they did playtest with a summoner, because Bahamut is summoned for such a short amount of time that there’s no guarantee he would ever be active at the time when the player left the dungeon. They might check that he’s behaving correctly within the dungeon, but not think of there being a problem when leaving.

I would have thought it’s no different to leaving any normal dungeon, and nobody’s had trouble with that up until now. Though perhaps PotD has the same coding flaw that just never got discovered because you can’t reach Lv70 in there, and any playtesting for HoH focused on new gameplay elements and not the PotD base programming.

In any case, I suspect it’s something they’ll deliberately test in any future new content modes...