Laguna II 1.9 dCi 107 Horses - Challenges with Cooling System Venting After Engine Overhaul

Laguna II 1.9 dCi 107 horses
Made head gasket, replaced rings, bearings, oil pump, engine is like a dream, but there is a problem with venting the cooling system. Warm air in the cabin appears randomly, after unscrewing the vent plug on the heater, fluid, steam, fluid again. The tank cap is tight, there is a lot of steam there.
Head after full machining, tightness control.
Any advice on where to look? New thermostat. The hoses are extra-hard, but the liquid does not boil over. Water pump (whole range) new, everything was done according to the art, my car and I personally put everything together.
I will add that I bought a damaged car, someone was already rummaging in the engine (fresh gasket under the head was)

I don't know if I remember correctly, but 3 vents are there.
1. On the thermostat housing, a nut like from a wheel vent.
2. Radiator the upper part on the left as you see from above such a large cross
3.A plastic plug at the entrance to the heater.
did you do that

Exactly. On the thermostat housing, key 14. Then the radiator, finally the heater. Is there any exhaust gas cooler or other system through which they could blow exhaust gas? dCi is mean to vent, but I'm not the first to do it and I haven't had such miracles. 3000 zlotys and a car pumped into the car
After unscrewing the cap, do not feel exhaust fumes in the tank. But in a smoky workshop I may no longer sense low concentrations ?

Pull out the thermostat, vent again and see how it behaves without the thermostat.

Okay, I'll let you know how to do it in 2-3 days, because there is no time ?. Thank you for your interest in the topic

Added after 9 [hours] 26 [minutes]:

I dismantled the thermostat and housing. I wanted to mount a new one but I decided to start the car. And a moment after firing you can see white smoke appearing in the head.

The head was machined by a company with many years of experience, nobody has mentioned pits that they can be significant. What do you think is the problem?