Bug - Invisible Unit(s) or events on the Strategic Map

Hello everybody, I hope that this post is conform to the etiquette.

While playing, I often encountered a pretty annoying (not really game-breaking) bug.

Sometimes one or more units become invisible on the strategic map. There is no way to interact with them (invisible, untargetable, unmovable, can't fight, can't join an army, can't explore, can't get engaged). They still appear on the side panel (the only way to select them).

It happened in different occasions and right now I have found only one way to reproduce it (I'm sure there are others):

-Get in a fight at the end of which you are supposed to GAIN a new army member ;

-Lose all army units doring the fight ;

- Win the fight with a summoned creature.

After the fight, you will still see a winning screen and you'll gain the unit like it should have been. Unfortunatelly, it will be invisible (and have all the traits I described before).

I have questions on that matter:

- Can I make a member quit my party (It is quite annoying to have to deal with untrained units)

- As anyone ever encountered such a problem?

Thanks