This picture is the area to notice when you follow my instructions.
This is a source file, you can convert to .aseprite if it matters.
-Note, I did test on both rotsprite and fastrotation.
reproduction steps:
frame 2
select tool(M)
mark all
rotate by 90 degrees counter clockwise
90 degrees rotation should be perfect for pixel art yet it distorts pixels. I’m on aseprite 1.2.4 but I noticed this before as well.