Strange behaviour when spacing LSB

When decreasing the spacing on the left side of my glyphs (with ctrl + arrow left) the letter makes a jump to the right.
When I save the file with the new LSB value, quit Glyphs and reopen, the LSB is back to the old value.

  • only LSB (increasing and decreasing)
  • stroke filter applied
  • no component
  • Glyphs 3.0.2 (3045)