It seems like offline history does not sync when back online. For instance, play history, and files “favorited” are not captured.
It is the case, but with 0 details like this it’s hard to tell anything 
See https://yatse.tv/wiki/debug-yatse-kodi-remote for how to properly report issues with logs.
Fair point. Attaching
debug.log (1.1 MB)
The log should begin with me being offline. I played a few songs, favorited a few, skipped a few. Eventually I returned to wifi.
Let me know if you’d like additional info. Thanks!
Ok so Plex 
Can you make a new logs with only 2 songs and be sure to fully read them? And give the names of the song you tested in that new logs.
I see Yatse trying to update some songs with playcounts and other without one but the logs when in offline does not give enough info to know what is played as the names are encoded.
For the favorites status I do not handle the offline status change for Plex for now.