There are a few Plex forum threads on this issue you might find useful for details… This one is around the TVs and has lots of references.
Typically most easily reproduced in MKVs with AC3\EAC3 audio (based on posts and experience).
A workaround that has been working for me is turning off Direct Play but leaving Direct Stream on in the Plex app settings on the Roku. This doesn’t typically trigger transcoding or any quality changes - just a little extra CPU work. I do get a little audio hiccup if I skip intro right away on a show but it’s fine after that (most likely I’m just getting ahead of the initial buffer).
This bug is present in local file playback on Roku Media Player and a few other local file playback apps so likely a Roku bug rather than Plex bug. It might help to report it to Roku Support (particularly mention the Roku Media Player aspect since that’s their app).
