I’ve gone back and forth on this too. At first I pulled mine out after every run because I didn’t want to risk the chemicals or constant water exposure damaging it. But after a while I realized it’s not always practical, especially if I know I’ll be running it again the next morning. These cleaners are designed to be water resistant, but I think the bigger issues come up if you leave them submerged for days at a time without a break, seals, plastic housings, and even the cord can wear faster that way.
What works for me now is kind of a middle ground. If I’m running back-to-back cycles, I’ll let it stay overnight and deal with it in the morning. But if it’s done its job and I won’t need it again for a day or two, I take it out, rinse it, and set it in a shaded spot to dry. That seems to keep the parts in better shape long term while still being convenient.
So I’d say overnight here and there isn’t a problem, but I wouldn’t make it a “live in the pool” situation 24/7.