Sounds like my issue was solved in the recent beta. Woop!
"When building spritesheets on export, Construct 2 has typically inserted a 1px transparent border around each image. This avoids color from an adjacent image on the spritesheet "bleeding" in and causing a fringe of a different color to appear down the edge. However this actually caused a different problem: in preview mode images work in a "clamp-to-edge" mode, meaning they show the last pixel's color right up to the very edge of the image. (Visually this looks like "hard" edges.) After export, since the edge of the image is now against transparency, it then means the last pixel's color actually blends with transparency up to the edge. (Visually this looks like "soft" edges.) In short, this means in some cases seams can appear between adjacent objects after export, because the edges are now blending with transparency at the edges, allowing some of the background to show through in between.
To resolve this Construct 2 now repeats the outside row of pixels from the image instead of inserting transparency around frames. This ensures that after export images blend at their edges the same way they do in preview mode, and helping prevent seams appearing. If 'Downscaling' is set to 'High quality' images are (as usual) padded out to power-of-two sizes for correct rendering with mipmaps, and now C2 also fills the entire padded area with the last row of pixels. This might look a little odd on your spritesheets, but it guarantees seamless rendering even with extreme downscaling."