On my Galaxy Note 9 and previously on my S8, syncing seemed to work irregularly. I just now manually refreshed my Inbox and emails which arrived at the server about an hour earlier were downloaded. Sometimes at the top of the folder list I'll see "Syncing disabled." But other times, without having changed settings it will download mail very promptly.
I have set "Sync messages from 'any time'", Folder Poll frequency Every 5 minutes, Background Sync "always".
I just now noticed that I had "Poll folders" set to "Only 1st class folders" and have set it to "All".
The behavior is as if the app is looking (as some other sync apps like Dropsync do) for some external condition like battery or network to be met, but today the batt is at 90%, and even when away from WiFi polling will sometimes work (maybe 50% of the time, maybe more).
(Overall, very much like the app, esp for its minimalist design sensibilities, a nice respite from the clown-car behavior of most of the rest of the stuff on my phone.)
--gvc