I’m pretty sure unions don’t support textures entirely yet as unions can consist of multiple complex shapes which the texture you’re putting on isn’t shaped to fit union. If you really want to put a texture on it, put it in blender and wrap the texture around the union. I would tell you how to do this, but I don’t use blender.
Good luck.
Is the Union a simple cylinder?
The Decal in your image that is showing up correctly, is that on a Cylinder Part?
Seems to me you are trying to make a snake-like rig. You’d me much better off using simple Parts set to Cylinder.
And how would your Union be over 10k tris?
Maybe show an image of just the Union by itself so we know what you’re dealing with.