That's what happened to me. And i guess that's pretty normal. Keybr adds a new key when the current focused one hits confidence level 1 and about 35 wpm. This means the currently boosted / trained key does pretty well, but might naturally suffer when another key / finger comes into coordination. Just because a key went well while focusing on it, it doesn't mean this will remain when proceeding with new keys. Your brain and muscle memory has a lot to do with learning to coordinate everything.
Looking at my all over progress you can pretty much see, that every time a new key kicks in, speed drops and errors go up. But this always reverses after a short time. I had keys that where at confidence 1, but later with all keys added dropped and took some time to get back to 1 or maybe still reside at 0.98 or so.
Just keep going. I think it's pretty much the way how it progresses. Maybe many can relate.