What could that bright flash on the space livestream be?

It may be an imaging artifact rather than something glowing in space. A bright source can bloom across a camera sensor, while an energetic particle can sometimes leave a brief mark on the sensor. The shape and duration matter: a tiny spot or streak lasting a frame is different from a glow that changes across several frames.

Reflections are another possibility. Light inside or near the camera can bounce off a lens or window and show up as a flash or colored patch. If the feature shifts or changes when the camera moves, that would point more toward optics than a distant object.

A genuine electrical or plasma-related event is possible in principle, but calling it plasma from a video alone would be a big leap. You would want independent clues, such as another camera recording the same event or relevant instrument readings at the same time.

I'd start with the original recording and check several seconds before and after the flash, frame by frame. Note whether it stays in the same spot on the image, whether nearby objects or stars are affected, and whether another camera shows anything similar. Without those details, there isn't enough to identify the cause reliably.