Getting to level 20 puts you at maximum cap. Losing a level means you are still at maximum cap, and will have to come all the way out of the red. Unfortunately you might need to wait weeks to see a difference.
AFAIK the Dev team are aware of this, and think it's okay. It's the "cost of doing business" when you're level 20. As someone who's undergone this process a couple of times, yes, it's rough, but it is what it is.