I have been taking a careful look at synchronization across devices with Version 6.3.4.
The good new is that synchronization is clearly happening.
However, there are still some fairly severe problems.
In the examples below, I refer to the device where I actually post my changes as Device #1 and the device I am checking for synchronization results as Device #2
Problem #1
Shows that I have started watching recently are appearing on Device #2, but the episodes I have marked as watched on Device #1 still are not marked as watched on Device #2 days later.
Example #1:
I watched all 3 episode of AI Confidential days ago and marked them on Device #1. Device # 2 shows that I have started watching the show and have added it to a personal list, but the watched episodes are not marked as watched on Device #2.
Device #1
Device #2
Very oddly, on Device #2, the show has the Watched Status highlighted though no episodes are marked as watched. Also, it has not removed the show from the Watchlist.
Device #1
Device #2
I have multiple shows that look similar to this.
Problem #2
When I mark an additional episode watched on Device #1, it will still not not be marked as watched on Device #2 days later.
Example #2
I marked episode 9 of Star Trek Academy watched days ago, but it is still not marked as watched on Device #2.
Device #1
Device #2
I have many examples like this.
Problem #3
One show - “FBI”, is listing more episodes than actually exist on Device #2. This may be caused by some sort of residual Trakt/TMDb inconsistency in the episode history for “FBI”, but I had thought there were no such problems for this show.
Example #3
Device #1
Device #2
By far the most prevalent problem seems to be that synchronization across devices is happening, but there is no consistent pattern as to when they happen, and many items take far, far too long to be synchronized.