The 16th channel of the preshower in the right arm (aka index 15) was showing a large pedestal. Looking at the scope, the signal was very noisy and independent of HV. Turns out the patch-panel plug had broken.
A BNC connector was attached to bypass the patch-panel, and the signal in the scope looked normal.
The signal for the 19th channel looked normal in the scope, so the pedestal issue may be the cable between the patch panel or the ADC card channel itself.