I believe the issue is (Beta Feature) Studio Solid Modeling Improvements
as I noticed the gaps between triangles in unions were finally getting filled, but I found this weird issue when unioning a block & two triangles onto the end of one block it messed up textures, where the side became the bottom, the top became the side, and so on.
When you Union or Negate 2 or more the Orientation of the Union is affected by which item you select first. (or last, it’s been a while since I needed to use this so if first doesn’t work then try last!)
Select the item you want the orientation of the final Union to follow.
Then add the other items by selecting them in the workspace or in the Explorer window.
Now Union the group of items. The orientation should still be the same as the first item selected.