3 HP and under, no derating is required.
Above 3 HP, derating is required because the "converter" section of the VFD doesn't have enough reserve capacity to supply the "inverter" section when the converter section is receiving only two pulses per cycle (as on single-phase) instead of six pulses per cycle (as on three-phase).
A redesign of the converte sectionr to add more capability to the diodes and the "bulk storage" capacitance could mitigate this problem, but then I would worry that the regeneration function of the inverter, which is controlled and protected by the VFD's firmware, might not work as well as expected (and needed).
