Blue glue…That’s what I had problems with on the intake manifold. It did not like glycol at all even though it was suppose to be compatible. This was in the 80’s and I don’t remember the brand but it would not hold a seal on the thermostat housing at all. Took the manifold off and got rid of all of it.
I vote for bad head gasket pressurizing the system. Drive it a bit with the radiator cap loose. If it doesn’t overheat/erupt, there’s your problem.
If the cap is removed cold and put back on then it should immediately pressurize if the head gasket leaks. Since there wasn’t any contaminants in either the oil or the radiator tank It’s probably not this but it would be a quick test.