alspal - ah, you're right. There's a quirk in C2's spritesheeting system where single leftover individual frames are cropped to their original size instead of padded to a power-of-two size, which creates an inconsistency in their display quality. However you can work around it by setting "downscaling quality" to "high", which forces all exported images to be padded to a power-of-two size. But yeah you're right, there are a few odd cases like this which WebGL 2 will fix. Chrome is already experimenting with WebGL 2 - you can experimentally enable it in chrome://flags, but note the C2 engine currently does not attempt to use it even when it's supported. It's a good sign that it should probably be supported some time this year, though.
FWIW, Construct 3 has a better spritesheeting engine that fixes some of these issues, even in WebGL 1 mode.