Okay, quick update - it seems that, short of quest progression, it doesn't matter if I travel about, close and open the game, or both in between any of the above steps. Aloth and Grieving Mother consistently lose their gear when I swap them out in the Black Hound Inn, at least at this time.
EDIT - going to Caed Nua, however, allows me to swap characters to my heart's content again, so I'm starting to wonder if this issue doesn't pop up in specific locations on and off. The previous time it happened, it was definitely after a party swap in Caed Nua.