UI elements not respecting sibling order

Hi there,

I have some UI elements with a background and text. When I spawn a new one I move it to be the last sibling with the intention of it drawing over the top of the previous ones. However I am not getting consistent results with this. It will work a few times, but then seemingly randomly will stop working, and despite being the last sibling it will draw underneath an element above it in the hierarchy.

All of them have the same Z index value and are all parented to the same object.

I am reusing them by enabling/disabling them if there is an unused one available. But I can't see why this would have any effect if the sibling index is still being set?

I'm using Unity 2017.4.14f1.

All help is greatly appreciated!