Lock text direction under Styles TAB of Main Collection

We have difficulty with the fact that the text direction unexpectedly jumps back to LTR after we have set it to RTL. When we import books with mixed directions or no-direction (in the Back Matter or Front Matter) in HTML this setting changes back though we do not want it. This is a ‘nasty thing’ for us and it feels a bit unsafe, because we have set the the direction. Therefore: is it possible to give an option to fix this set direction?

This is happening to me too in SAB 9.1.1, in the book collection the Text Direction is set to RTL, every time I add a book this resets to LTR.
This is of course fairly easy to fix, but it would be nicer not to have to do it every time.
The ‘books’ I am adding are Word documents, the text therein is all RTL but I don’t think there is way in Word to set the document as RTL or for SAB to know.

Do you have the Interface Language on LTR?
I am not sure this will help, but at least I have it turned on (but I am not yet using version 9.1.1.