So this was fairly tragic… not sooner than I had posted my question I went through the steps I’d outlined above and saw an error in my material.
Gotta love how this didn’t get reported in the compile/build process to give me a heads-up… sigh…
In case anyone else has the exact problem…
When I had right-clicked and created the material, I had set the Material Domain to User Interface, but what I also needed to do was set the Texture Sample node to use Linear Color instead of it’s default, Color, for its Sampler Type.
Once I had changed this the error was removed, I clicked Apply, Save and then re-packaged the project. It all worked as expect at that point.