I see it ALL the time, across MANY domains.
Language, music, golf, programming, driving, competitive gaming, etc etc.
It's not necessarily a bad thing; it's WAY more effort to push for improvement. Once you've gotten to the point where your skills are serving your needs, is that what you want to invest your finite energy into? Maybe not. God knows I'm not actively trying to improve on every skill I have. Very few. Most of my things (music, games, sport) are just to have fun. If you're having fun you're probably not really improving, and that's ok.
But when people lament that they've hit a wall on a skill, in my experience it's this effect, MUCH more than any other.
I think if OP reflected on their already MASSIVE achievement of becoming functional in another language, they'd likely conclude that their skills rapidly increased up until the point that they had a functional level of the skill, and then hit a plateau once they subconsciously began expending less active effort on improvement.
Trever Moore's follow-up "Supersize Me, With Whiskey" was better.