Hey everyone!
Since the last update, I’m having an issue with outline in Convolution Matrix. It didn’t occur every time and I’m not sure whats causing this, but when I delete all the invisible pixels outside the art and try again, the outline works fine. I’d checked with color picker if any of those pixels have a little color, but everything has the code “#000000 A0”.
Hi @LuisLudic, would you like to share your image with me at support@aseprite.org so I can give a try?
Other possibility to do an outline now (as v1.2.9 was just released) is using the Magic Wand tool and the Stroke option (S key) from Edit > Stroke menu:
Sure! I’ll send you the .ase file
Thanks @LuisLudic, I’ve just checked your file.
This is a known issue in how some filters work with completely transparent pixels (pixels with Alpha=0). And the outline convolution matrix has some problems when the RGB values are not black for pixels with Alpha=0.
With the Color Curve we can check the real color of transparent pixels:
See that there are some hidden non-black pixels. To fix this we can use the Magic Wand tool to select all transparent pixels, and press the Delete key (Edit > Delete menu):
Something that we need to fix in a future version in some way.
Thanks for replying, David! I’ll use that Magic Wand method until this bug gets fixed. 
The next version of Aseprite v1.2.10 will have an option to draw the outline automatically in Edit > FXs > Outline
