Diagnosis
Biofilm and mold in hidden areas the clean cycle doesn't reach. The primary culprit is the drain pump filter housing and the rubber door bellows (gasket).
How to Fix It
Run a hot water cycle with two cups of white vinegar, not detergent. While empty. Then, clean the drain pump filter. Open the small access door on the front. Have towels ready. Unscrew the filter cap, let water drain into a pan, and rinse the filter. Wipe inside the filter cavity.
If That Didn't Work
Mold is deep in the door gasket folds. Lift the rubber seal back and meticulously wipe all sides with a 10% bleach solution. Then run a clean cycle with an affresh tablet or bleach.
Heads Up
Always leave the door and detergent dispenser drawer slightly open after every wash. This lets the drum dry out and prevents the smell from returning.