Hey there, Joseph!
It can be a bit complicated, especially if you're dealing with lenses that don't have a stabilization switch. With lenses that have no switch, the IBIS will be on or off based on your last setting. If I remember correctly though if you haven't changed the setting, it will be on by default, and you will have to manually turn it off in settings if you don't want or need it. If you save a custom setting that has it disabled, it will stay off when you turn the camera off and back on as long as you save the custom setting with it off. A lens with a physical stabilization on/off switch will always override this, whether it's third party or native.
In that case, it's all or nothing the lens switch controls both the IBIS unit and the lens stabilization. If you'd like to use only the ibis, you can do so by using a dummy canon ef-rf adapter, but you will have to input your focal length in the settings manually, and you can only focus the lens if it is mechanically focused - focus by wire lenses will not be able to change focus with a dummy adapter. Most older ef lenses do have mechanical focus mechanisms, though.
The mysterious disabling is perhaps because the lens switch was set to off, perhaps? Or if they are not stabilized, could it be because you have the IBIS set to off in a custom mode?
I always have 2 custom modes that are identical, except one has IBIS on, and the other has it off. That way, I don't have to fiddle with the menus to turn them on and off.
I hope that helps! I'd be happy to comment on any other questions you might have as well!
-Kyle
