Currently, there’s a known issue with Facebook periodically disconnecting accounts that should not be disconnected. They are working on this bug, but have not shared a timeline on when it will be fixed. In the meantime, please try this workaround and let us know if you’re still having problems.
Example of what you should see. Notice the 'deprecated' account on the left.
2) Click Add Page/Group
3) On the subaccounts page, there will be a profile account branch with the account name and one with a "deprecated-" prefix. Under the profile account branch with the proper account name, select the subaccounts to connect. Don't worry if it says it's already connected to parent account. Scroll down and click 'Connect Selected Pages/Accounts'
4) On the main accounts page, confirm that the subaccounts have switched over to the FB account without the "deprecated-" prefix.
5) Delete the "deprecated-" account.
Please let us know how it goes, and feel free to reach out with any other questions.