In this
pan test project I noticed that the pan automation points are not accurate. In Track 2, move the edit cursor to 1.1.00, there the automation point has the value "center". In the goniometer you will see that the pan value is slightly off to the left. Now move the cursor to the second pan automation point at 1.2.00, the pan value there is 100%L. In the goniometer you can see that the value is not 100%L. Check also the other automation points with the cursor and the goniometer, you will see that the automation values are not correct. It is not possible that the goniometer shows 100%L or 100%R when the pan mode is Stereobalance or Stereopan. The pan mode "REAPER 3.x balance (deprecated)" shows 100%L or 100%R in the goniometer.
If you check and uncheck "Linear scale above -3dB pan law" in the Pan Law settings in Track 2, you will also notice an unwanted volume difference.
REAPER v.6.61 x64 Win10 x64