Undo should be used (if you wish to) immediately after you use a tool. So having it still enabled after using another tool may have caused confusion.
Many thanks to those who have helped me improve these tools. Your support is appreciated!
I'm happy to say the next version is already finished with 3 new tools, I'm just waiting for the online store to get things set up. Hoping for early March at this stage.
Until then, v1.6.2 is available here.
Update: I just released patch v1.6.3 that has another improvement to the Undo code.
If you like my tools, please click the G+1 icon on this post and/or my blog's sidebar to let me know ;-)