Had it now several times and in the last case even reproduced it.
After prolonged combat the initiative order gets stuck with a single unit.
I have tried to tame a larger group of spiders quite a bit below my level, so I just soaked the damage while waiting for tame to come off cooldown, with the rest of my army waiting in the background.
At some point I get stuck with one of my units - so far always one that I have tamed earlier in that battle. The unit appears repeatedly up to the top of the bar with only one other unit shown at the very top. Whether I pass, move or attack, it is just added back and I can't get to any other unit anymore.
EDIT: Just had the first time a combat of similar length (long enough to use tame 3 times) where the initiative order did not break. Can't really tell what was different this time. The total number of units, turns and moves seemed to be roughly the same (or more).