Could it be that you are setting the default value to "A", so by clicking on "A" you are not actually "changing" the value. If you select "B" or "C" first and then select "A", it would probably work?
Perhaps you can set the default value to 'null', that way clicking on "A" would actually be a change.
So, I removed the default value of the attribute which seemd to have fixed the issue. However, the issue has now returned without any changes to the model. When clicking back and forth between the 3 options the option A is passed correctly occasionaly but fails in most cases. This suggests some kind of actions are conflicting and the order of execution influences the outcome. I have checked for changes of the attribute but there are none except for the microflow which initially calls the page