My initial attempt at pinpointing the causes of the 'floating' pion mass examined, as possible causes, the fit function, an artificial increase in the opening angle, and the BEMC energy resolution. Preliminary findings indicate that the both the opening angle and the BEMC resolution play a part in the phenomena, however the resolution seems to be the dominant factor in these pt ranges (5.2 - 16).
I also compared the effect in Data and MC using a full QCD (pythia + geant) MC sample, filtered for high-pt pions. As can be seen in the link below, the mean mass position and peak widths are comprable in data and MC. The floating mass problem is readily reproduced in this MC sample.