But early DLC vehicles work just fine and can be sorted as everything else. That's because they were included in the game files from the start?
The garage limit seems to be researched thoroughly, at least according to the wiki. It claims that you have 64 slots shared between all types. So you actually have only one garage, different access points just have different filters on type. Unlockable vehicles are ignored in this calculations. All this is consistent with what I am observing myself.
The game only stores information about this many player-procured vehicles. If you go to save editor and add 70 vehicles, the game will then just cut the last 6, you won't see them. Now the mechanism to prevent the player from storing too many vehicles seems to be separated. It just stores a number. When you add car, it increments (or recounts all list, needs testing). When you delete or lose it (give away your car in theft activities), it decrements (or recounts). If it reaches 64, it gives you warning about garage limit and prevents you from storing anything new via interface means.
So. If you go get yourself 64 cars, the headcount in the game will be 64. If you go delete some of them in the editor, the game will still think you have 64. You'll have to delete something from within the game to free up this number or it'll keep telling you that the garage is full. I didn't test it yet, but I assume the opposite is also true: if you add cars via editor and hit a limit, there will be no warning in-game, but new vehicles will be lost while the game tells you how you stored them successfully.
___
Disregard all that. Testing showed that it doesn't work. Everything is fine. Strangely, yesterday it seemed like a pretty clear bug. I remember deleting a few vehicles in a save editor and getting "garage is full" afterwards, while clearly being able to add more via export/import (and it showed in-game). Maybe game just needs player to drive out of the garage zone and get back to recount vehicles. I did that now and everything was right as it should have been. Sorry about that. Yet again testing dominates theories.