I agree with the perception part of things. I know Kris talked to a dev who confirmed this suspicion but I do remember back in league 7 that my team had a trash run of form going from all Ws to DLLD and I went to switch tactics and saw my captain was still assigned back to when I let the b string 11 play against a significantly weaker team, so technically my starters were functioning "captain-less" in a way. After I reassigned to the proper captain (usually my GK because of Mauricio Pochettino) my form returned. Definitely could be coincidental, I'm just saying it's odd.

But I usually assign GK as the GK has total vision of the field and could communicate to his line anything he sees wrong in formation, which could be passed up to the midfield/strikers up front. I always imagined if an outfield player other than a CB were the captain, they wouldn't have this view over the game and not really know what call is the right one to make. Also, they are the easiest to access for the manager as they aren't moving constantly so anything I would want to tell them would be best told to the GK.

But then again I'm an overthinker at heart and I used to assign ML and DL to my AMLs so they would dominate their flank (no real difference, btw just a waste of time lol)