DIAGNOSIS Mildew is growing in the wet internal parts you can't see, primarily the hose and recovery path. Emptying the tank alone doesn't clean the system.
FIX You need to deep-clean the entire fluid path. After emptying the dirty tank, remove the nozzle (the part that touches the floor). Rinse the dirty tank and its filter with hot water. Then, with the nozzle off, pour a cup of white vinegar into the dirty tank slot. Tilt the machine forward so the vinegar runs down the internal hose. Let it sit for 15 minutes, then reassemble and run clean water through the system over a sink.
IF THAT DIDN'T WORK The filter in the dirty tank's lid (a gray foam square) is a smell magnet. Remove it, wash it with dish soap, rinse thoroughly, and let it air-dry for at least 24 hours before reinstalling.
HEADS UP Never store the machine with liquid in the dirty water tank or with the brush roll installed while damp. Always run a clear water rinse cycle after use and let all parts dry separately.