I've finally managed to replicate this annoying bug. Like http://www.byond.com/forum/post/2525288, it happens since the new icon editor was added.
Descriptive Problem Summary:
Image editor corrupts icons when using Undo and Select tool.
Numbered Steps to Reproduce Problem:
1. Create an icon with 4 dirs.
2. Switch to EAST direction.
4. Select a part of icon and delete it.
5. Select another part of icon.
6. Switch to NORTH direction.
7. Press CTRL+Z to Undo.
Editor switches to EAST and Undos.
Editor switches to SOUTH. The SOUTH icon state is replaced with EAST icon.
Nov 26 2019, 4:20 pm