Many thanks guys for letting me know that the issue is likely related to Vallejo primer, I will be very careful with next usage of masks in this build.
Fortunately I've got decals for all roundels and stencils required.
About the nature of this primer, there are no specific indication on the can but it's almost surely acrylic.
I wanted to test it because it's convenient, aprox 16 Euro for 4 400 ml can, in comparison with Tamiya that, in the same shop, cost 12.20 Euro and it's only 180 ml.
Alberto