I've got a recurring problem with our Intune Company Portal running on iOS devices. It seems that after a period of time (the last two instances happened about a month apart which has been highly annoying for all involved), our company email on iOS devices will stop syncing. The only fix I've found that works is to remove the Exchange/Office 365 account from the device, and then re-run the Device Verification in the Company Portal app. Once that succeeds, the user then usually has to go back to Settings > Passwords and Accounts at which time they are prompted for their O365 password again and everything starts syncing (until the next time this happens - which can be a few months).
This doesn't happen on Android devices - they are fine.
I've double-checked our setup and can't find anything out of the ordinary according to the setup instructions. So I'm running out of ideas.
For simplicity sake, I am not running anyone on MFA (although I want to enable that). And our APNs Certificate is valid for a few more months.