Triggers on Hidden Layers Still Firing

Alistair​ Strangely enough, I hadn't noticed this before. However, I often use layers as “trigger reserves” when I don't want to clutter up the trigger panel on the base slide.
But the important clue is that the trigger fires “when the variable changes” (immediately and wherever), not “when the layer timeline starts.” 
So it's just a different logic, which can be used in other circumstances to achieve immediate and dynamic actions without adding more and more conditional triggers. For example: adding points to a score = ONE trigger on the slide mask (add 1 to Score when var changes) for the entire module.
But for updating old modules, this must be a pain.
In your current case: perhaps by moving the variable trigger on the layer (not visible) to the first position. Changes in the state of objects on the base slide do not modify the variable but display the layer. (Sorry if this suggestion is not appropriate, I may not have fully understood all the issues in your case).