11-07-2019 09:51 AM
Its true, the flash cannot be used while recording movies. Page 163 of the 90D's Manual
To confirm, I would recommend resetting the camera to default setings and with the flash down, change to Movie Mode. If it pops up, you might have found a bug. If not, you might have changed something or in conjuction with other settings (like exposure compensation) which caused the behavior. Should be fairly easy to determine.
11-07-2019 09:53 AM
Do you have "disco flash" focus assist turned on? That might be why it is popping up. Try turning it off.
So funny LoL! Thanks KV got a good chuckle before walking out the door.
11-08-2019 09:58 AM
Bug, not defect. Since the camera is new, I'd contact Canon. They could fix in a subsequent FW release.
They won't however, if they don't know about it or its not reported.
11-08-2019 03:44 PM
Even though the flash can't be used when shooting video, is it possible that the auto-focus assist beam (or "disco flash" if you prefer) can be enabled as a focus assist for low-light video shooting?