Hello @Bert,
At first sight, there are things missing from mask MSK_DETFOO_*.gml files, they have only 1KB (I also noticed this in the screenshot from @Jan), and their content is just something like:
It should have been something like this instead:
Therefore, the extent polygons are missing, and this type of mask (MSK_DETFOO) is per polygons. This is why SNAP is not picking them up.
@Jan, do you have any idea why these MSK_DETFOO_*.gml files are like this?
Thank you in advance.