This is most likely normal behavior from red-eye reduction being enabled, not a battery or flash fault.

With red-eye reduction, the flash fires one or more pre-flashes before the exposure. Those short flashes make your subject’s pupils contract, which helps reduce red-eye in the final image. After those pre-flashes, the shutter opens and the main flash fires for the actual exposure.

Check your D850 flash mode/settings and turn off red-eye reduction if you don’t want the extra flashes or delay before the shot.