You are sure no water has come in, or do you think?
I race a lot in the wet and I know from many years experience if you do not kit all openings, water will come through.

A simple test you must do is switch it all on and when there is contact switch of the transmitter, if then the car goes on full throttle (put the car on a stand with the wheels from the ground) then the cause is a failsave issue and still it can be just a small drop of water in the receiver.

But what a weird setup, An open ESC and the receiver in a box. OK, the manual says the ESC is waterproof, apparently it is not.