MFA gives the option to save a device for a number of days (max 60) so it doesn’t ask for validation constantly.
Unfortunately, it doesn’t request a renewal of the validation until a new file/folder is created locally and tries to sync. Note that until this moment it won’t download new data from the cloud storage to the computer either
Once a new file/folder has been created, it will show a window saying OneDrive isn’t signed in (see attached image) and will ask for the credentials to start syncing again (account’s password and MFA code).
This has been reported to Microsoft who said they are working on it. They also reported MFA can’t be removed just from OneDrive, it only allows to be enabled or disabled and it affects every Microsoft service/application.
Signing out and back in from OneDrive doesn’t create a new session. It uses the previous one until it expires. At the moment (as per 30/09/20) there’s no workaround. I will suggest creating a reminder on a calendar and create a “dummy” file/folder to force this process to happen.