Fix position when scrolling rearranges layer order

I’m suddenly getting this odd behaviour when I select “fix position when scrolling” for let’s call it object A.
Object A, suddenly moves itself to the bottom of the layer stack.
If I reposition it where it was, “fixed position when scrolling” deselects itself.
If instead of moving object A to the top, I move all the other layers under A to recreate the original stacking order, then “fix position when scrolling” remains selected.
I’m also getting strange behaviours when viewing prototypes. Objects that are under layers in design mode appear on top in the prototype.
It’s all doing my head in as there appears to be no logic to any of this!