Doesn’t matter if you use retargeting or not.
The engine does whatever the f*** it wants.
Look up the skeleton retarget settings. See what they are set to. See what changing them does for the blendspace.
There’s just too many reasons.
Most of them are the engine doing what it thinks you want it to do. Poorly.
I guess that’s why being a “character animator” is a profession though, right?
As far as the AO - they use a different (but similar) process under the hood.
As far as the IK bones.
Tbh, I mostly use then to place Items and animate them.
Foot ik bones are actually perfect to bounce a ball around the animation. Just socket it to the bone and animate it however you want without needing to add additional bones, change the skeleton, etc.
It’s also super useful for reloads that require a foot (crossbow) but are dynamic in nature (length of cross bow is variable).
I usually also add an explicit Hat and an explicit Item bone just to have - the hat one particularly as you can then use it with creative weight paint to move the cap (or part of it, like the helm visor) up and down.
We digress from the OP’s issue though.
It’s kind of impossible to know the exact reason without looking at the setup and trying things.
And just because it works for me out of the box in .25 with custom animation and proper skeleton settings does not mean it would work in .26.
You’d be hard pressed to see the difference when it’s running anyway.
The point of the IK is that it toggles on/off gradually with the steps.
Because it’s gradual, you won’t notice it being “off” when it’s active.