Weirdness in Spain?

Just started playing in Liga BBVA for the first time, it's a long-running career save, so it's 2026 in-game.

Can someone understand how the main teams and the 'B' teams relate to each other? It's obviously not the same as the reserves I've been used to playing in the British Isles most of the time. Also, there's something odd about squad numbering: it says I can have a max of 25 players registered (OK, that's easy), 19 and unders don't need a number if they're that age on Jan 1 of the year (OK)...but I ended up with two players UNregistered, even though I definitely went through and made sure they had numbers. One of those players I had tried to register repeatedly, only to keep getting his number stripped off after the next match.

So, what's going on here? Why is the game unregistering my players, without notifying me or asking me? Is there some extra rule that I don't know about?

I've heard of a bug where a player suddenly gets unregistered without notice, and even during the non-registering periods.

But you should check whether you have too many foreign players or not.

There is a rule which states that you can have a maximum of 3 non-EU nationality players. Perhaps that's it?

And as far as I understand it, the B and C teams are separate teams. They have their own coaches, training, signings, etc. They play in a different league, but the players can move freely from B to C to the actual squad.