I've had the same issue. I had to move all the other tiles equally away from the edge. Then select them all again and move them back to the edge.
I've also experienced this as well when zooming out, then zooming back in. This is a new UI issue, I've been on DT for over 1 year and only recently has this started to happen. I'm not doing anything new that I haven't already done in the past.
Plus, no matter what it takes to make this issue happen, it never should in a robust UI. So it's definitely a bug.
If you could, I'd suggest opening a support case for this. It would help us all out. I have too many UI support cases going at the moment.
For me the trick that almost works all time for this issue is to change the browser zoom to minimum, its also helps with the issue when you try to move a tile and its become red even it is not hovering any other tile or border.