As I am seeing this problem as well I can add some input.
I see it on iOS 13.3.1, OS-X 10.15, Windows 7, Ubuntu 18.04 LTS, Android Pie
All running the latest Enpass version (I checked for updates when I had the issue)
All devices update time automatically
The last sync time is not displayed (blank) and there is no warning about unable to sync like I see when using WebDAV
I don't have a screen capture at the moment
Like Casual User the iCloud login screen from OS-X has no "keep me logged in" check-box, but from Windows and Linux is does have the check-box and I have it ticked, however the connection has to be re-established every few hours on any of the platforms I use.