Found a workaround I can live with!!!!
Get the old Plex Classic channel: https://www.rokuguide.com/private-channels/plex-classic
In the audio settings disable dts, but leave ac3 enabled
Dts will now play as Dolby digital and play in sync!
Not a perfect solution, as I do notice a different quality difference between DD and DTS, but I can live with this for now.
No. Doing that was causing Plex to convert to AAC 5.1 which the roku downmixes to stereo pcm.
If you have a file you can upload to our problem media box account that shows this dts audio sync problem, please pm me and I'll send an invite.