The reason for the phenomenon is that at wide apertures in good light necessitating a fast shutter speed, most of the exposure is coming from a very narrow traveling slit between the first and second curtain.
The curtains before and after the slit are not at the same height, however.
The electronic curtain travels where the photosites are.
The mechanical curtain travels at significant distance from the sensor. In particular, there is an anti-aliasing filter and perhaps some protective glass on top of the sensor, and only then the shutter curtain.
Because the curtains are at different heights from the sensor, parallax effect becomes significant. The faster the shutter speed, the smaller the traveling slit is, and the problem becomes more pronounced.
To fix the issue, the shutter speed needs to be slowed down. Firstly, if the camera is not already at its lowest ISO setting, the ISO should be lowered to the lowest ISO. Secondly, if the shutter speed is still too fast causing the effect to appear, an ND filter could be used.
However, the best fix is to use shutter curtains that are at the same height from the photosites. So, if the second curtain is mechanical, the first should be mechanical too. Unfortunately, with all cameras, this is not possible.
Here is a diagram showing how the parallax between curtains affects the light coming into the camera. The black bars represent the shutter/curtains (electronic or mechanical) and the blue bar represents the image sensor.
- On the left, both the first and second curtain are at the same depth; no matter what angle the light enters at, the slit between them appears to be the same width.
- However, on the right where the curtains have different depths, light entering at a downwards angle is nearly blocked while light entering at an upwards angle effectively sees a wider slit.
The different angles of light are associated with different parts of the bokeh, so one side of the bokeh will get dimmer and dimmer until it is eventually cut off entirely. Also, the narrower the slit is, the smaller the angle at which light is blocked; this is what makes the effect more pronounced at shorter exposures.