Can't answer the rest of your question because I don't use a one plus but:
aren't you supposed to charge the phone overnight?
No, you aren't "supposed" to charge your phone overnight. Leaving your phone on the charger at 100% is actually pretty bad for long term battery health. Hence why the notification exists in the first place. Modern phones also full charge in like an hour, so this leaves your phone in that state for many hours.
The longer story is it's actually best to stop charging your phone at 80 percent unless you really need the extra juice, because any time your phone spends above that is potentially damaging, but that tends to be hard to deal with for most people.
Most of the phones I've seen with this feature have a "battery warning" or "charge notification" or "protect battery" type setting somewhere you can turn off. But again, I've never used a one plus so Idk if they do or where it is.
Yeah, we all know where the priorities really are.
How have our consumer protections gone so fucking far.