I can understand why the transform tool generates sometimes slightly kludgy 90 degree and 180 degree rotation, but I don’t think there’s a good reason why the rotate menu option does as well. There’s no reason for even heights/widths not to rotate perfectly at 90 and 180 by transposing as opposed to performing RotSprite or FastRotation. Odd rotations may still be slightly off, but it’s got to beat some of the problems I’ve run into where single pixel stuff simply vanishes.
I think that the problem is probably that you had “rotsprite” selected while you used the rotate menu option.
If you have “fast rotation” selected, 90 and 10 degrees rotations work perfect, no matter wether you applied them using the menu or using the transform option and the mouse + shift.
Also, welcome to the Aseprite community! 
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