Not entire sure what else can affect the rotation of this Toe IK bone, but both in edit and pose mode it has no 'roll'.
But something that is strange it has World-Z aligned axis shown in Edit mode, whereas in Pose mode it shows its Axis rolled by -90°. Though when selecting the bone it states that no rotation is being performed on it.
Does anyone know of what else might affect the bones roll/rotation in such a way?
Hi Pascal,
This will be a great problem solving exercise for you :)
If there are no constraints on that bone and it's only affected by its parent - take a look at the parent.
Check if there is any accidental transformation on the bone (pose mode), and also check the constraints (toggle them on/off to see if it changes the orientation)
Then continue up the hierarchy hunting for the cause.
This might seem annoying but this will build your skills and your understanding really quickly.
Of course, if you're still stuck you can upload your file as an exercise and I'll give you a hand in fixing it.
I admit I had spent a further two hours earlier to figure this out.
After going back and forth a bunch between bones to check parenting / constraints settings / drivers / etc.
I found that the parent's (Toe_IK_parent) 'Copy Transforms' constraint is the one causing it.
I was able to 'fix' it by setting it to L,L instead of the default W,W space.
But I must admit, I have no idea why this is doing it.
No other Copy Transforms constraint needs to be set to L,L in the entire setup.
And the fix is nothing more than a patch until I know whats causing that behaviour.
I checked the bone that it was Targeting (MCH - Heel_roll_02) and it is neither rotated, rolled, nor affected by its parent or constraint, to explain what the driving factor is behind this behaviour that I could see.
(Lets ignore the fact that I had assigned the wrong widget to the two Toe bones, in case it was confusing in the video).