This looks like the classic mis-timing between curtains movement and the flash duration. You probably know all of this but I will repeat for those who may not know.
For shutter speeds faster than the camera flash sync speed (typically 1/200), the camera second curtain starts moving almost immediately after the first curtain…so the light hitting the sensor at one time is a swath of light (curtain opening) smaller than the size of the sensor. Multiple swaths of light (as the two curtains traverse) will hit the sensor to make up for the whole image. This is fine if you don’t use flash (natural light). With flash since the flash duration is so short, only one or two swaths of light are lit up by the flash, you end up with a lighted band as shown in your picture.
Normally Canon prevents this from happening by two things – 1. Limit the shutter speed to 1/200 or slower and 2. Allows High Speed Sync on the flash.
In High speed sync mode, the flash emits a series of flashes matching the swaths of lights so everything is properly exposed. What I don’t get is how you were able to bypass # 1 without the high speed sync. I’ve been trying to do this for my teaching materials without much success.
Theoretically, it is possible to get this with shutter speed slower than 1/200 for mistiming of strobes duration and triggering…what you need to do is to adjust the flash duration and timing of the trigger.
