Definitely sounds like a clocking issue. There was a recommendation at one point, when using Dolby Audio Bridge alongside DigiLink hardware, to create an aggregate device in Audio Midi Setup so that DAB and the DigiLink hardware could share a common clock. But I thought the need to do that had been eliminated in subsequent DAPS versions. How to set it up was in one of the Pro Tools Expert 6 part Atmos Tutorial articles.
Here is a link to part 1. I'm not sure which of the 6 articles this setup is in.