AHA! Actually this was not the root of the problem. I found other problems with non-looping animations and replication, and the ultimate fix to all of the problems was to be specific about the section name to play the montage from. I think this might be an ability system bug rather than a core bug, but it seems unable to correctly identify the section by default on a non-looping animation, and supplying it (it's normally just "Default"), all the problems go away 🤔