I am not an expert in DC offset, but the track you posted is good stereo and according to the code at https://forum.audacityteam.org/t/dc-offset-trickery/26935/4 when made mono, there is minimal and variable DC offset which would suggest to me that it’s occurring naturally.
So unless someone wants to contradict me I don’t think there is anything to be concerned about in that waveform.
If you think there is a problem you could experiment by swapping over the right and left ends of the cable or use an alternative output in your chain (you have not given details on exact connections you are using). Also check you have the moving magnet/moving coil switch set correctly on the Pro-Ject Phono Box.
Gale