make sure there is no moisture on the usb/phone connection. As with the phone make sure there is no moisture there. I suggest you put the phone somewhere to dry out ( even if you think it does not need it)
I have noticed this in the early morning sometimes. I leave my usb lead in the car and if I plug my phone in the morning and it's a cold evening I will have the alert. it is sometimes triggered by the cold also. I think anyways. 👍
This happens with my S9 regularly. I think it's a battery issue as it will happen if I let my phone go completely flat, then try to charge it, it can sometimes take days for it to go away. Or if I let it get very low >5% then try to charge it with a standard charger rather than a high speed one. My guess is that it uses a resistance reading to check for moisture but this resistance can also be present in a faulty battery, the higher ampage of the fast charger somehow gets around this.