DIAGNOSIS That rubber seal ring around the underside of the lid is a notorious trap for tiny food particles and moisture. It's not dishwasher-safe for a reason—the heat can warp it and bake smells in.
FIX You need to manually deep-clean it. 1) Carefully pry the clear plastic lid apart from the black rubber gasket. They snap together. 2) Remove the rubber ring. 3) Scrub it thoroughly in warm, soapy water with a small brush (an old toothbrush is perfect). Get into all the grooves. 4) For lingering smells, soak it for 30 minutes in a mix of warm water and either white vinegar or baking soda. 5) Rinse completely and let it air-dry fully before snapping it back into the lid.
IF THAT DIDN'T WORK If the smell is truly baked in or the rubber is cracked, you need a replacement. This is a common wear part. You can order a new lid seal (sometimes called a "blender pitcher lid silicone gasket") directly from Ninja's parts website or other retailers. Make sure it's for the BN701/Professional Plus pitcher.
HEADS UP Prevent this by cleaning the gasket after every use. Don't just toss the whole lid in the sink; take the extra minute to pop the ring out and rinse it. Storing the lid loosely on the pitcher, not sealed tight, also helps air it out.