I noticed that I get the "not updated" status on a fresh install until I manually perform a scan (even just a "flash scan" will do - and it only takes a minute or two). Even after manually updating Immunet, I noticed that closing and re-opening the GUI (or restarting the computer) resulted in the "not updated" status. The only way to fix it was to update then scan.
On every machine I've tried so far (all some variant of Windows 10), the following seems to fix it:
1. Open Immunet GUI
2. Manually check for updates. Wait a few minutes, to give the ClamAV database a chance to update, then close the update dialog.
3. Now run a flash scan (or full scan, but beware a full scan can take hours whereas a flash scan only takes minutes).
It seems as if forcing an update then forcing a scan resets Immunet's indicators and it all works again.