Diagnosis
That musty smell is mildew growing in hidden, damp areas the main wash doesn't hit. The usual culprits are the rubber door gasket, the float chamber (under the lower filter), and the drain hose loop.
How to Fix It
Beyond the cleaning tablet, you need to manually clean the hidden spots. Wipe down the entire door gasket, especially the folds, with a vinegar-water solution. Then, remove the lower rack and filters. Look for a small, round plastic cover near the front—that's the float switch chamber. Pop it open and use a paper towel to soak up any stagnant water and debris inside. Finally, leave the door slightly ajar after every cycle to let it air out completely.
If That Didn't Work
Check the drain hose connection under the sink. It should have a high loop (secured up under the counter) to prevent sink water from flowing back into the dishwasher. If it's lying flat, that can create a pocket of smelly water that sits in the hose. Also, try running the hottest, longest cycle (like "Sanitize") empty with a cup of vinegar placed upright on the top rack.
Heads Up
Using rinse aid actually helps with this. It improves drying, leaving less moisture behind for mildew to grow on. Make sure your rinse aid dispenser isn't empty and is set to at least level 3 or 4 in the settings menu.