It's been an issue for a while. It's caused by the fact the game doesn't actually download the maps from the Workshop, but from EA's mirror server, to allow people who bought the game on the EA app to also access them, since they can't get it from Steam.

But the synchronization process seems to occasionally get stuck.

The best thing you can do is report it on the Discord of the developer, Petroglyph Games, on the C&C Remastered channel there. They know who to contact on EA's side to kick it back into gear.