I've experienced this phenomenon several times as well, but can't say I would attribute it to any significant change in engine speed. It
does seem to have some relationship to energizing/de-energizing the A/C clutch, but the tach doesn't really "flinch" when it happens. IMO, an engine speed change would take longer to change and settle at the new rpm than the relatively short length of time the pedal seems to soften. The first few times it happened, I thought I'd been gently bumped from the vehicle behind me at the light, but quickly realized it had to be something else when that other vehicle was still approaching or not there at all!
It
feels more like a momentary reduction of vacuum assist to the power brake system. Not sure what the relationship is there, since the A/C clutch is operated electrically, not vacuum assisted. It's been a while since the last occurrence, so I may be compensating by subconsciously adjusting my old ways to adapt to the new vehicle.