I have a similar issue. It seems to happen arbitrarily but now it happens more often. I have a two monitor setup and I've tried pushing the icons to the right (secondary) monitor which is arranged in portrait mode while the main monitor on the left is in landscape mode. Both are at 3840 x 2160 ( or strictly speaking the portrait one is 2160 x 3840). The fences are locked but often (but not always) when I log out and return the fences move. It also happens most commonly when I play Age of Empires 3 on Steam and then I log out and log off Win 10 without shutting down. When I log back in the fences are all on the primary monitor now and when I try to restore a snapshot, it keeps the fences in the primary not the secondary monitor as I want them.
This happened before but not too often. Now it's happening more regularly.
Sorry to hear you are having issues. On issue about multiple monitors please review these two guides :
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