Whenever I make this simple shape with a plane and unwrap it, it unwraps into a weird shape:
I know I can unwrap with project from view or cube projection to get the correct shape so I'm not after a solution per se, I'm looking for an explanation about what's the cause and why it happens as well as it it's a bug or not. from what I've tested it happens whenever there's a long rectangle attached to another one, as the shape get bigger and become longer the uv start to twist in weird shapes,
- I did apply the scale and it's not the issue here.
- The corners are exactly 90 degrees
- the scales are 20cm*280cm with 2cm extra width on each side at the base for 10cm if you want to make that exact shape
Edit: as you can see, a shorter version unwraps correctly as well as the one I changed some of those 90 degrees angles: