You put or have coolant in the system. When you run the engine the coolant heats and expands. Creating coolant system pressure. When engine cools down the coolant returns to the original volume. . This should not allow full system pressure after it does.

I might test the coolant for exhaust components. I have not played around with enough 123 engines yet it is logical to me. Pressure testing the system may show nothing. Cracks in heads are not totally unknown on the 123 engines.

Once again I am not saying this is the issue but perhaps it should be checked. I probably would before trying to shotgun for the issue. Also look for bubbling in the system with the cap off. Careful though and perhaps just starting the engine cold and let it reach operational temperature. Or since the engine may not be reaching atmospheric boiling temperature it should not blow coolant out. People can get scalded if not careful remember.

Also unequal cooling can be created with the by products of combustion getting into the water passage and forming a type of blockage or bubble in the fluid. Again I would test the coolant for exhaust components. One reason is it probably is the cheapest thing to do. Plus it clears or condems easily what the cause may be.

I also have no issue with if you post it was something else later. As it certainly could be. I just did not want you to ignore this as a possibility.