I am not complaining, merely reporting what I have observed in case it might be helpful to others.
I have noticed with my EOS R5 that firmware updates change IBIS behavior when I am using non-Canon teleconverters. Before updating to 1.9.0, my EF100-400mm f/4.5-5.6L IS II USM +1.4x III would work with my Kenko TELEPLUS HD C-AF 2X DGX , but after the update it does not work unless I switch off IS on the lens before turning on the camera. With the IS switch in the on position, it sounds like the IBIS is slamming itself from one extreme to the other. The Monarch butterfly was firmware 1.6.0.
But, a combination that did not work before now does work. EF100-400mm f/4.5-5.6L IS II USM + Kenko TELEPLUS HD C-AF 2X DGX + Vivitar SERIES 1 1.4X AF. The Kenko reports a change in focal length to the camera and the Vivitar does not so the IBIS is not quite right even though it is working as a result of using the incorrect focal length. The focal length is reported as 800 mm even though it is actually 1120 mm. The F Number is also reported incorrectly.
A third experiment, EF100-400mm f/4.5-5.6L IS II USM +2x III + Vivitar SERIES 1 1.4X AF did not work for me today.
https://www.rsok.com/~jrm/2022Oct07_birds_and_cats/2022oct05_monarch_IMG_0664c.html 
An experiment today Symphoricarpos orbiculatus ( also called Coralberry, Indian Currant, Buckbrush ) firmware 1.9.0 :
An experiment from 2021 with a different combination and firmware version 1.4.0 is at:
https://www.rsok.com/~jrm/2021Sep08_birds_and_cats/2021sep02_bumblebee_IMG_3837c.html
