Continuing from:
https://forum.glyphsapp.com/t/bug-glyphs-mini-pasted-pixel-components-are-added-over-previous-ones/13636/5
As mentioned in previous post, if copy-pasting pixel components between letters, the Pixel tool gets confused if those pixels are flipped, see GIF:
EDIT: Shown here, when I click, it deletes pixels from a different coordinate:
to reproduce, copy-paste a a group of pixels between letters, then flip them.