Thanks for the detail here! The irony is that Due has been syncing notifications much more reliably in the past few days since I posted this. Haven’t changed anything in my set up of workflow except for not using Due on my iOS 14 beta testing iPad, and acknowledging notifications more often before they go into auto-snooze. If the latter has helped the sync situation, there’s a knock-on positive behaviour adjustment… 
Also, thanks for making Due. I’ve used it on and off for years now, and there’s nothing else that quite compares. It’s one of a couple of apps I’ve known about for some time, but that I’ve recently come to appreciate how well they actually meet my needs.