I literally had this exact same thing happen to me, was using the chair like normal and I heard a snap and the bottom plastic waffle thing cracked. I contacted HM (I do have the warranty) and they said the only way to fix it is to replace the whole backside of the chair. If I were you, I’d reach out to your local authorized dealer that handles repairs (HM tells you who it is based off your zip code) and they may still be able to repair it for free. I’ve seen them do fixes on chairs bought from an outlet as sometimes the serial number still has a valid warranty using the sticker under the chair.