I had the same problem. I was fighting a neighbor, happened several different fights.
I think because some spots count as X.5 movements, so if unit's movement is 6, and one wire or whichever block counts as 5.5, so you move to that block, but I think the system rounds it to the 5, showing you can do one more move, but when you try to move again, that becomes 6.5 moves, which is more than what the unit is allowed.
The numbers are only an example. I haven't had a chance to prove my theory yet, so I might be completely wrong as to why it's showing you can move one more block.
Only happens on the new map when fighting with or against Progressive Age unit.s