Cannot add custom fields in models anymore

Hello, I'm reporting the following problem:

I used to add several custom fields to my models (simply through clicking to edit->add new) into the model view.

For some reason, when I try the same now, the fields that I create are labeled as BASE FIELDS (instead of, of course, CUSTOM FIELDS).

See pic:

To be clearer, I can even create fields without the "x_" prefix.

These fields newly created are invisible (I cannot use them in any way, nor seeing them into the DB via PGAdmin). Furthermore I cannot delete them once I created them since they are seen as base content.

What's happened, and how could I fix this?