We study light emission properties of a population-inverted 2D electron-hole plasma in a quantizing magnetic field. We observe a series of superfluorescent (SF) bursts, discrete both in time and energy, corresponding to the cooperative recombination of electron-hole pairs from different Landau levels. Emission energies exhibit strong renormalization due to many-body interactions among the photogenerated carriers, showing pronounced red shifts as large as 20 meV at 15 T. However, the lowest Landau level emission line remains stable against renormalization and show excitonic magnetic field dependence. Interestingly, our time-resolved measurements show that this lowest-energy SF burst occurs only after most upper states become empty, suggesting that this excitonic stability is related to the "hidden symmetry" of 2D magnetoexcitons expected in the magnetic quantum limit.