Flag lists visible for one authenticated user but not for others
I found a solution to this problem, although I can't say I fully understand why it was happening. Seems to be a case of too many relationships in the View, causing some type of conflict that appeared only in some cases. During the building phase I had added a batch of 6 relationships, trying to get it to work but ended up with more than were necessary. I eliminated them one by one and tested, until the truly necessary ones were identified. It really only needed 2: "Flag lists: User" and "(List user) Flags: User's flagged content (current_user_flag)."