The gasket that was installed is from Glaser. The mechanic sent me the instructions sheet and the torquing values and procedure is exactly the same as in RAVE, so that excludes a bad installation.
Today it was tested for gases in the coolant and the result was positive

So, even after testing the head 2 times and checking out OK I'm thinking that it's definitely cracked somewhere. Maybe a hairline fracture that isn't showing up.
For now, the game plan is to get another head and test it again.
Is there any definitive way of checking the head ? I remember reading something about a UV dye that was introduced in the coolant and would show the fractures under UV light, after removing the head. Does anyone know anything about this ? I'm just finding it hard to invest in another head without being absolutely sure that the problem is there.