If the Re-Gain feature is used with "LR/M" selected, the faders of LR mix and Mono mix are each moved for compensation, just as one would expect.
However, the faders for Send to Mono mix are not seen as absolute values, but relative and coupled to the LR level. So when the LR fader position is changed for a signal, it already affects the signal's Mono output level, there is no need to change the Mono fader, too. So the Re-Gain actually overcompensates for the Mono output.
I just started using the Mono mix bus so I'm not 100% sure, but Behringer's X32 EDIT app for PC shows that the Mono fader only acts relatively to the LR fader, in both panning modes (LR+Mono and LCR).