I think your recording track is a stereo track (even though it can have the input set to a mono in, it will record on only one channel of the stereo pair).
In this case your ‘Pan’ control isn’t actually a Pan but rather a ‘Balance’ control - hence the effect you’re getting.
Look at your events in the project window do you see a waveform only on one side of a pair?
You can’t change the track from stereo to mono so you’d have to create a new mono track for recording in future.
You can also, in your present case, change the Pan control to a Stereo Combined Panner (see pg. 186 of the manual) and set for summing of the channels.