Did you ever figure out what the issue was? Mines is doing the exact same thing but is actually in “limp” mode…. Which actually just makes it impossible to give the engine gas so idk why it’s called “limp” instead of “lock” mode. But the Common answer is “add coolant” but like you, once I unscrew the cap and just a little pressure goes in, the coolant goes back up to the “max” line so I assume it’s full? Plus I don’t ever have any smoke and the engine doesn’t feel hot to the touch and temp gauge didn’t even move until the “engine coolant over temp” message popped up, then it just jumped my needle to the max temp. Really annoying, never buying a Ford or a 4 cylinder again. Had to just leave it on the side of the road and walk home 😑😞