Looks like something similar to Plume Impingement to me.
If you notice, the "flash" looks like some form of gas, and appears prior to what I believe was an attitude maneuver, as the direction of the camera footage changes abruptly afterwards.
Depending on the thruster technology, it requires priming before injecting fuel/oxidizer in the chamber and igniting it. So the gas may be the priming leaving the nozzle before the ignition that executes the maneuver. Priming could be the fuel itself without oxidizer depending on design.
Plume impingement is normally an effect that concerns the deposition of material or degradation of material on satellite surfaces, and a solar panel would be an example, but a thermal coating would also be of concern. It also causes undesired torque, which may degrade performance of wheel desaturation by thrusters.
As JohannesD noticed, it occurs right after backshell separation, which could mean some compressed-gas mechanism that jettisons the backshell, or the thrusters correcting attitude after the separation, which would likely cause some disturbing torque, the priming hypothesis explain why the flash appears before the maneuver rather than after.
Sorry about the somewhat speculative answer.