BMW e39 2.0i - Replaced Fluid Expansion Tank, Losing Coolant (0.7L/100km) Without Visible Leaks

Hello, I recently cracked the fluid expansion tank, I bought a new one, I replaced it. It turns out that now there is less fluid. The car leaves no traces, drips, etc.
Also:
-the thermostat holds 85-90 degrees
-compression on each cylinder around 16 bar
-head test done on the head gasket (the liquid did not stain, i.e. approx.)
- don't smoke
- no butter under the oil plug
- no oil comes
- heating works very good
- the heater cannot be seen leaking under the rugs
- the car has power, acceleration as before
- fires without problems
- after changing the fluid I vented on a hot engine.
- you do not see that the liquid is bubbling, mixed with oil, etc.

After warming up the radiator hoses are quite hard, but they can be pressed about 0.5 cm.
I will add that the fluid is running out quite a lot. At 100km about 0.7 liters.
After warming up the car, it can be seen on the tank that it has gone, but after unscrewing the cork, it hisses and rises. But on the longer route, after cooling, you can see that it has dropped significantly.
I wonder if the fluid filler cap could be guilty in this case? Would the signs show that?
How to check if the radiator is ok?
Regards.

As you wrote, the cork can sometimes let go, especially if there is still air in the system.

Cork ordered, tomorrow is to come. This morning I see that there is almost no fluid in the pouch. I unscrewed this little plug for venting at the thermostat, it hissed and fluid appeared above the tank Would this indicate an infestation?

This engine is best vented when the front of the car is a little higher than the rear, on a cold engine, not hot, only the ignition is switched on, not the engine, temperature at 32 degrees climatronic, full air flow.