Sorry for the lacking response, I’m really not sure what is causing that issue and I’m not sure what the best way to get more info to help you is. It really looks to me like something is fighting with Smooth Sync, but I don’t see what it could be with your setup.
Generally what I recommend for mysteries like this is to try and recreate the problem starting from one of the built in project templates, that way you can let me know the exact steps to recreate the issue so I can investigate.
Barring that, the only other way I’m going to be able to investigate is if you send me your entire project, or a stripped down version that demonstrates the issue.
Sorry I don’t have a more straight-forward answer for you.