It has occurred with Edge of Tomorrow, Guardians of the Galaxy, as well as an old DVD, and I had my BDP set to output LPCM. This happened with my old Denon 2809ci, and a firmware upgrade fixed the problem (on the Star Wars BDs).

The difference is that the audio dropouts on the Star Wars films were at specific points in the presentation,. and rewinding would get you the dropout in the same place. These dropouts are random, and rewinding doesn't repeat them.

I use a panny BDT460 that will output LPCM, and el cheapo Sony and Samsung players that output bitstream.

JSS