I have imported several xml files from Sibelius and always when I had a rehearsal mark F it was imported as a rehearsal mark and the dynamic sign of forte “f” below the rehearsal mark.
Maybe you could change the way the rehearsal marks are input — I mean numbers instead of letters. I’ve had no problems with numbers imported from Sibelius (even numbers until 154 !)
Unfortunately MusicXML does not describe textual elements in documents in a particularly semantic way, so it can be very difficult to determine whether something is a dynamic, a tempo, a rehearsal mark, or indeed many other things.
But still Dorico should not interpret one thing twice, should it?
MuseScore has no problems importing my Sibelius xml file correctly. And this is freeware.
So I hope this will get better in future.