Hello, I'm only on FM20... but, unless these issues have been specifically fixed for FM21, I'm assuming they may still exist as I'm having trouble finding reports of it on google. So here's the problem:
Note: This pertains to B-Teams only that appear as affiliates to the parent club (not Reserves/u23/u18/etc).
1) First-team coaches that exist at the start of the save do not appear in the B-team's coaching assignment screen.
However, any newly hired first-team coaches will appear in the B-team's coaching assignment screen.
I'm unusure if the intention is for them to appear or not, but the above behaviour is at least what's true.
2) B-team coaches that exist at the start of the save are the only coaches that appear in the B-team's coaching assignments screen.
As a result, any newly hired B-team coaches will not appear in the B-team's coaching assignment screen.
3) The manager of the B-team never appears in the B-team's coaching assignment screen.
It doesn't matter if the manager was there at the start of the save or if you hire a new one. The B-team manager is never available to be assigned to B-team coaching assignments.
4) If the B-team has maximum amount of staff and you are given the 'transfer blocked' screen where you can choose to replace a member of staff, the button confirming your decision doesn't work for the B-team. You have to manually terminate or mutually terminate the staff member's contract, otherwise the new transfer in will forever stay in your 'transfers in' page.
Sometimes, even when terminating the contracts yourself, the 'confirmation' screen refuses to show up and you are forced to cancel the transfer via the Transfers page. You can easily check this by hiring a new B-team manager without terminating the existing manager's contract beforehand.
You can easily test all this on a new save:
Just load the Bundesliga only and start as Bayern. Choose a late pre-season start date so that the B-team isn't on holiday, then delegate your staff hiring duties to yourself and hire a first-team coach. He will then appear in the B-team's coaching assignment.
Then hire a B-team coach and watch as he doesn't appear at all in the B-team's coaching assignment.
The issue isn't specific to generic coaches only, it happens to assistant managers, fitness coaches and goalkeeping coaches too.
Here's a small example:
Hiring a new senior squad coach with no secondary job (Michael Adam): https://i.imgur.com/nLV2iT8.png
He's clearly part of the senior squad: https://i.imgur.com/GCS1eju.png
Yet he appears in the B-team coaching assignments despite none of the other senior coaches doing so: https://i.imgur.com/mZ09JEh.png
Once again, I'd just like to point out that this is an issue for B-teams only, not regular reserves or u23. B-teams are listed as affiliates in your affiliate page and are usually named something like Bayern II, Dortmund II, Barcelona B, etc...
I've already confirmed this in FM20, I just don't know if it happens in FM21.