The connection between the two must fluctuate, and each time they reconnect the screen lights up...
There are hundreds of variables. When the screen lights up, does it say "wirelessly charging 50%" or similar? If it does, the phone thinks it has just connected to the charger and commenced charging. Mine will do that if I disturb the device whilst sitting on the pad...
And you have the stay awake while charging turned off in developer options?
If you've had software updates, a cache partition wipe is always a good idea to clean out the debris.
Settings can also slip after updates, so a wander thorough them is useful.