Keith,
First, thank you for the link to that post. It’ll be a useful resource to me going forward.
The observed tail was indeed caused by the scintillation photon energy distribution as demonstrated in the graph below with the red data and top axis now represents only optical photon count absorbed in the tracker, not energy. Curiously, a test I had made counting the number of absorbed photons before changing the optical photon distribution to a Dirac delta still showed the heavy tail. My suspicion is that the absorption of the lower energy optical photons was the cause in that case.
Thanks for your assistance! Hopefully this thread of conversation is helpful to others in the future.