You need to perform a coolant pressure test on your engine plain and simple.
If the pressure stays the same after a couple hours, good news.
If the psi goes down, first check for any hose leaks around the engine compartment. If everything checks out there, it’s likely a HG leaking coolant into a cylinder which would make sense if it’s hydro locking upon startup. And time to tear down that engine. This would also be a good time to get cylinder heads checked at a reputable machine shop.
If you used the correct pressure test kit that can be found at Autozone (free rental), you wouldn’t have to block off coolant reservoir tube and would probably get a much more accurate reading on what’s going on.