Hey,
I mostly saw the problem when hosting, indeed, and it may well be the case that I had a match saved in Training Club. In the beginning I didn't realise I could play while waiting, so did not have matches saved, while later on I almost always had an ongoing match there. Since the problem seemed to get worse with time, that may be the reason. I more rarely joined matches, but had issues with desyncs then as well. Another idea that came to mind is whether the Xbox 'Quick Resume' option has something to do with it.
I haven't been playing much online lately as I am very much engrossed in a 2021 World Tour season with a young Sinner (and so far I tend to be regularly trounced online--I seem to have more issues with timing strikes right, probably because the opponents hit faster than the Pro-10 AI I'm used to), but will keep an eye on this when I come back to that.
On a different note, I noticed that on the World Tour sometimes the day of the week seems a bit odd in relation to the stage of the tournament, like second round matches on Thursday and things like that.
On a yet different note, I've watched some recent videos about the current TE4 competitors, and that brought to mind (once again!) how amazing it is that you've single-handedly succeeded in doing something big studios with big money failed terribly at all these years: creating a great, true-to-life tennis game
