One of mine did this; it’s a very common problem. The newer koralias have an “anti-reverse” feature on the cage that’s supposed to prevent the issue but mine had broken off in regular use (less than 6 months).
You might be able to fix it with a dab of cyanoacrylate. *Carefully* take off the cage to see if anything has broken off or cracked inside. Turn off to pump first of course!
The other option is to try and contact them directly and ask for a new cage w/ the anti-reverse feature. Good luck with that; they totally ignored me when I tried to contact them.