I am Alexia from the Asana support team, nice to e-meet you! 
I have looked into the thread and thanks for all the information you provided.
Given the fact that Asana seems to behave the right way with stories (we would usually advise to wait at least a couple of hours before undoing an action so that the story is kept on the task), it seems that Zapier is not catching the new event on the task even if it is recorded in Asana.
For this reason, I would recommend contacting Zapier so they can check and look further in their system to understand why the new event coming from Asana did not fire the Zap.
Could you please keep us in the loop once you have more information?
If Zapier gives you an update involving us checking in our system, do not hesitate to contact the Asana support team (https://forum.asana.com/t/how-do-i-contact-support/19057/7?u=marie) so we’ll be able to investigate further for you, Martin.
In the meantime, have a nice day!