This looks familiar somehow...
What we do know is an outline effect works by coloring transparent pixels if there are pixels around it that aren't transparent on the texture.
Maybe since the effect samples pixels far from the pixels on the sprite it could be sampling pixels from other sprite sheeted images.
So maybe the effect isn't clamping the sampling to to just within the current sub-texture.
If that is the case then either the effect could be modified, or we could take advantage to how the c3's effect compositor works to avoid sampling beyond the frame (sub-texture). You could do that by adding another effect to the object, and order that to be before the outline effect. When there are multiple effects the first effect is drawn to a temporary texture and that is used as input for the next one. With that nothing would be beyond the image to possibly sample.
Anyways, just an idea.
Huh... after some testing it's an issue with construct? For one of my objects I ended up both removing the effect entirely as well as removing it from the family, the object has 0 effects or behaviors, and it still looks like this.
Which once again, the strange outline changes depending on if my page is full screen or not and is visible in the editor despite the sprite not having any such lines.
Edit: I have now also moved the objects layer around as well, still didn't change anything.
Edit 2: Have now changed the object's sampling to the 3 other options, the only one that got rid of it was "Nearest"... but that one made it look ugly.