BUG ? Weird issue with master text frames

Hello,

I'm having a really weird issue with some master pages.

Normally, any text frame inside a master page will be showing with dotted lines in thumbnails preview, right ? Well I thought so.

I have a master page with one text frame (Master page F), that is based on master page A, which doesn't have any text frame, only footnotes.

When applying Master page F to any page, it gives me editable text frames, which should not be happening, since it is a text frame related to master frame ! And to confirm : those text frames appear with continued blue edges in thumbnail previews.

However, when I use a normal master page, composed of one text frame (master page B), not linked to any other master page, it gives dotter blue edges when applied to a page, which is normal.

See this screenshot :

What is going on ? Am I missing something ? This seems like a weird behavior to me

Thank you