Loom banner preview is inaccurate

The bug

The loom banner preview shows lines that aren't actually on the banner.

How to reproduce

1. Put a red banner in the loom
2. Put red dye in the loom and select the cross-shaped pattern
3. Observe the banner preview
4. Take the banner and place it in the world
→ The banner preview was wrong

Notes

This is a trivial example for easy reproduction, but this can cause issues in making banners appear to have seams that aren't really there. More examples: