The problem is cross-image bleed in spritesheets when downscaling (because the smaller mipmaps are lower resolution, and combine areas of the image in to single pixels). It's a normal result in computer graphics when using spritesheets and mipmaps, it's not specific to Construct.
The 'Downscaling quality' property was added to Construct specifically to solve this. It pads images on spritesheets out to power-of-two offsets which largely avoids the problem. The reason it's marked as "not recommended" is because this makes it use more memory, and so you shouldn't choose it for no reason at all. (I'd noticed in the past people choosing it because they "want better quality graphics" without understanding what it does or what the trade-off is, so I added the "not recommended" label to discourage that usage.) You should only use it if you really have the problem with seams, which you do, so you should use it.