Rares...
Maybe I can help you out.
I was able to solve that same issue in my game.
What I did write down on my Dev Diary:
Good news: I got the 2x and 1.5 scales mode to work fine on my game.
Dont know exactly what was going on, but I may have a clue.
I did create the game using Stencyl 2.2.
There I was not able to have 2x scales in a correct way. It was really bad and lost all
resolution as I did state before.
Then I did migrate to Stencyl 3.0.
And instead of testing 2x scales again I did try 1.5x scale.
But as my game was created on Stencyl 2.2, it didnt have the 1.5x image on the resource folder.
So I did import my character image again and did try 1.5x. It was perfect.
Then I did try 2x scale... perfect again.
So, all I had to do was to re-import all images again. no big deal.
Only 1x that is not so good, but its regarding Stencyl not being so good to resize the
image to 1x. Will try later to resize on photoshop and overwrite them.
And here it is an before / after images:
Before

After

(I have to re-import the menu / pause file)