Engine

Get a block test carried out .

Agreed.
you can even do the dye test yourself.
Ebay sells a combustion leak tester kit for around £30 or so.
Small beer to rule in or out.
Simple to operate,
load the tester with blue fluid and place the tester in the tank.
If the fluid changes colour then there's combustion gasses in the coolant.
So head or block fault.
Other causes of you problem could be a blocked part of the coolant system near a cylinder/head.
This will cause a super heated area which could cause an air lock just like you get in a house heating system.
Poor circulation from a bad coolant pump or blocked water jacket.
Or there was this one job I had in the same as your,
actually worse than yours!
This van when hot blew steam from the heater matrix so bad the entire van interior ended up like a sauna!
That's a lot of steam!
After some unusual testing using a compression leakage tester with each piston at bottom dead centre and the cam out so all the valves were closed so pressure testing the entire height of each cylinder did pressure leak from cylinder #3 into the coolant.
And after stripping the head off and lowering #3 piston to BDC could a hairline crack be seen.
The important part about where the crack was meant it was pressuring the coolant well before TDC when the diesel would have been injected so a block tester using the dye tool wouldn't have shown a colour change.
So,
Block test using the dye tester,
If no colour change the a compression leakage tool used to pressurise the cylinder after the cam has been removed to make sure all the valves in the head are shut and the leakage pressure testing done when the cylinder being tested is at

Woops sorry hit send before I was finished.
So a cylinder/block leakage tester uses fluid that changes colour from blue to green or yellow depending on which fuel system is being tested,
They're around £30 from ebay and you could do this test yourself.
A compression leak back/leakage tester is around £50 upwards and also uses compressed air from a compressor and you would need to have the right adapter for your engine and for a full cylinder test the cam would have to be removed so the vales were closed and the cylinder being tested was at BOTTOM DEAD CENTER.
Probably best if a good garage performed this test.
Nether of these tests will fix your engine they will only isolate where the fault is.
The van I quoted needed ended up getting a new engine but because of my unusual pressure leak back test I was 100% confident telling the owner just what was wrong.
And to be honest I took the head off when he gave me the go ahead for an engine replacement.
I just had to see inside,
It was a goalpost shaped crack about 40 mm up and the same across,
about 70 mm down the cylinder.
Good luck finding a mechanic crazy enough to do a pressure block leak back test at bottom dead centre!