How effective is R6's IBIS with EF Lenses?

I have noticed a similar effect with a couple of Powershot cameras. With IS enabled, it seems that the first shot of a series is more prone to one side or corner being inexplicably soft. The cameras in question are the S95 and G1X original. These cameras only have lens-based IS, but I believe that the softness could be caused by the IS element being at the extremity of its travel at the moment of exposure. It seems more prevalent at longer focal lengths, where presumably, the element has to move over greater distances.

I haven't tested exhaustively, but the effect seems more likely to happen when using 'continuous' rather than 'shoot only' IS, and due to the way that compact cameras tend to be used, the first shot of a series has usually been the first shot after turning the camera on.

The series are usually either focus or exposure brackets. The three shots are triggered with a single press of the shutter button.

While 8-year old compacts with lens-based IS may seem to be far removed from a top-end mirrorless camera with IBIS, the commonality is that their IS systems are both controlled by firmware written by Canon, so I can't help thinking the problems may be related.

To try to avoid the problem, I have started doing the following:-

1) use shoot-only IS rather than continuous

2) use a 1s self timer to avoid motion from the shutter button press on the first shot

3) turn IS off when not needed.

These measures seem to be helping, but I don't have enough to go off as yet to be sure.

If there is something special about the first shot after turning the camera on, then another measure would be to simply fire a single shot before doing the bracket sequence.

Anyway, this offers no solutions for the problem discussed, but I was struck by the similarities. Instances where stabilisation systems may actually degrade image quality are seldom discussed.