Since 3.11 dropped, whenever this person plays with another player or group, they never see the party markers and the others can't see theirs. Is this an intentional change, or is this a bug?

It's a bug. We know about it. We're trying to fix it. It might not be fixed. I know it's not fixed for 3.11b. But it's something that we are trying to fix and trying to get to the root of. I think it's been happening for the past year. One of our coders took quite a long time looking at it and trying to get to the root of the problem, but wasn't able to get to it. They're going back to it.