When Mother Superior is found slain at the lab upon returning there (stage 220 of the Ambition), there is a mandatory action that is supposed to remove the Laboratory Services of both April and Mother Superior. This only happened for April, but not for Mother Superior. However, number of workers in the lab is reduced by two points.
This resulted in the bug above. The third slot is actually Mother Superior’s card, but it is invisible. The third card slot remains vacant and cannot be filled.
UPDATE: It seems that there is some kind of server lag in updating an account’s data? The quality for Mother Superior’s Laboratory Services is gone from my player character at this time of writing.
UPDATE #2: The bug is still there. I have an invisible card that I cannot discard.
UPDATE #3: Apparently, my dude only has enough qualities to pull two cards from the deck - no other card. In other words, this was not a case of invisible cards.
edited by Rostygold on 4/14/2020