While me and @BigGoldieIsReal were working on a game, we found something that we think it hasn’t been mentioned here before.
Also this is my very first post here, let alone a bug report. Keep that in mind 
Explanation: Union faces bug out when union-ed with another part and don’t apply textures, decals, etc correctly. Mostly divides them in half or only applies to only 2 faces.
Visual Aids: Check reproduction files.
Direct Links: Not related.
System Information: Not related.
Beta Features: only Studio Solid modelling improvements is enabled.
Rep. Files:
– Create a cylindrical shape with 4 decals on each face
– Make the cylinder an union (can be alone or with any other part) and the decals, textures… Will stop applying correctly. Even with applying the textures after
Expected behavior
For the decals and textures to apply on the 4 faces differently, not only on 2.