Slide Timelines trump Master Slide timelines
I am using Master slides for lots of things. I was curious to find out which layer of slides (a regular Scene slide versus the underlying Master Slide) takes precedence when changing the value of a variable.
To test this, I created a Text variable called "showMe", and set it's initial value to "initial value". I then created a trigger on the custom Master Slide that was set to fire when the timeline reached 2 seconds, and would change the value of "showMe" to "Master". I also created a similar trigger on the regular scene slide - a slide that made use of the custom master slide - and had this trigger set "showMe" to "Slide". Then I ran a preview.
"showMe" got turned to "Slide", which means the scene Slide trigger fired last - or at the very least, the order of precedence was Master Slide, then scene Slide.
So if you're trying to set variables on scene Slides, and use global functionality/triggers or tracking on the Master slide, you might want to put in a fractional time delay into the Master slide triggers so it has time to register the properly updated values.