I've posted on this several times. With my vehicle it occurs almost every morning... decending a steep hill with cold engine. If you shift to "S" your meter will come back! I often have this "problem" with a no-regen issue (stepping on brakes OR decel results in no green bars for a few minutes... even with 70 degree temps and mid-level SOC).

I've accepted this as a daily recalibration.... and, it's true... it DOESN'T affect your overall MPG. I've seen the MPG meter go up even when the instant gauge reads zero!