Based on viewing the slow motion video you posted:

Something is causing the first shutter curtain to reopen as the film winding lever reaches the end of its travel. This is a malfunction that should not be occurring. Your camera needs to be examined by an experienced camera repairman familiar with 1970s era Canon cameras. Good look finding one in 2020.

The Canon AE-1 has a horizontally moving set of fabric curtains that begin with the first curtain covering the film gate between the light box and the film. When the shutter button is fully depressed to take a picture the first curtain opens by moving to the left (as viewed from behind the camera). At the interval prescribed by the selected shutter duration ("shutter speed"), the second curtain closes from right to left to cover the film gate. Note that the curtains always move at the same speed across the film gate. What determines exposure time ("shutter speed") is the interval between when the first curtain begins to open and the second curtain begins to close.

When the film winder lever is moved the shutter curtains are also reset by moving from left to right. They overlap slightly so that no light is allowed through the film gate as they move. The second curtain disappears to the right as the first curtain reappears to cover the film gate opening.