Diagnosis
Dirt or debris is jammed under the throttle paddle, or the return spring is failing.
How to Fix It
Clean it first. Power off. Use a soft brush and isopropyl alcohol around the base of the throttle paddle. Work the throttle back and forth to loosen grime. Let it dry completely.
If That Didn't Work
The throttle assembly needs replacement. It's a single unit. Order a Mi Electric Scooter throttle. You'll need to open the handlebar console to swap it.
Heads Up
A sticky throttle is a major safety hazard. It can cause unintended acceleration. Do not ride until it snaps back reliably every time.