Why is CiviRules not firing when the trigger is adding a tag?
Thanks for sharing your problem.
As you can see there are a few outstanding issues with tags: https://lab.civicrm.org/extensions/civirules/-/issues?scope=all&amp;utf8=%E2%9C%93&amp;state=opened&amp;search=tag
As you are probably aware CiviRules is open source like the rest of CiviCRM. That means that issues will be fixed in a number of ways:

organizations run into bugs and get their own staff to fix them
organizations run into bugs and pay a CiviCRM partner/developer to fix them

So if you are in a position to either fix the problem or fund someone else fixing it that would be great! Most of the times issues do not get fixed without someone bearing the cost at some point, the power of open source is that the solution is then shared freely.
On top of this CiviCooP sponsors a yearly 2 day sprint to fix as many as the issues as we can in 2 days. But there is no guarantee that the issue that is bugging you is on top of the list (although in this case it probably will be....). This sprint is going to be in the week of 10 May. Any sponsorship for an additional day will be welcome :-)