Exchange 2016 slow circular logging

Would anyone perhaps know how exactly does circular logging work? I know the theory, of course, but would need to know the following:

• is it working all the time (e.g. 24/7) or does it stop at any time due to a specific event task?
• when mailbox migration is underway... does it run at all times or does the mailbox need to be fully migrated first, and then circular logging does the job of playing into DB and truncating logs

The issue we are seeing is that circular logging does not catch up with mailbox migrations from one DB to another and the log files drive gets filled up to 100% (despite circular logging enabled) which cause databases to fail...

Thanks, 

G