I handle hundreds if not thousands of accounts, Facebook has changed a lot of issues since then but there's always bugs if you're unlucky enough
This is not totally true, as OP said they can advertise through the app, they can very likely add people,
I've had many people with their personal ad account being blocked and asking for verification, they can just ditch that and use someone else's
Facebook can either ban the ad account, ban the whole BM, or ban your personal account from using the BM, depending on each situation there's solutions.
After the restrictions are applied, there is no way to add people to the business accounts.
Facebook is part of a dominant market and cannot simply block a company (advertiser) without strong justification.
I remind you that in these cases, they block personal account that has never been used for advertisements, but keep active business accounts and advertisements, which shows that nothing has been infringed, but no longer let the person manage the business account and advertisements, this is illegal and unjustified.