Thanks Frank,
After digging into the problem, I found out the problem wasn't actually JS, but rather it was the Critical CSS I was using... I created a Stackoverflow post on it ( see here )
Apparently the critical CSS of the second sub page was different to the first and as a result was causing a problem...
Thus, I was wondering if you can have different critical CSS for different pages...
If not, do you think it would be a good idea to simply create a separate header file in my theme and manually the critical CSS for the sub page there...