Thanks for your reply !
I indeed scale it to have the line.
After several tests, I found that I have this problem when I’m setting this decal as a child of another actor (same problem with DecalComponent) which rotates (even if I set the attachment rules to KeepWorld when attaching it in C++).
By manually detaching it and forcing its rotation to (0,90,0), I have the same results as you !
Now I need to find why my AttachmentTransformRules are not working (or if there’s a problem with my rotations) 