We found the beam quality factor M² of the fundamental mode as a function of wavelength is U-shaped in the working photonic bandgap (PBG) of an all-solid PBG fiber (AS-PBGF) for the first time, to the best of our knowledge, and our simulation results also match well with the phenomenon. The normal band that is near the high-frequency edge of the third PBG integrates the lowest M² and single-mode operation simultaneously, while the other two edge regions suffer from anomalous variation of M² versus wavelength. The general applicability of this finding can be further extended to other PBGs and also other representative structures in the AS-PBGF field.