Hi there, toroperro!
Thanks for reaching out to the Apple Support Communities! It sounds like MyFitnessPal is not transferring information correctly to the Health app. To start, let's make sure there are no pending updates with MyFitnessPal. You can check out this article to Update app or use automatic downloads and be sure MyFitnessPal is completely up to date.
If there are no pending updates, and your workouts are continuing to show incorrectly, I recommend connecting with MyFitnessPal app developers to continue working on this by following the steps in this article for How to contact an app developer.
Take care!