@givemeawhisky Thanks so much for looking into this in such detail. I’ve just published the site for a moment to check if the code works and it does!
I’m sorry to cause a bit of a false alarm, I was just a bit nervous about publishing the site as it’s not mine (work for a client) but I received her permission to do so earlier today.
Like you have discussed still seems a bit of a mystery as to how the 2 preview modes differ from each other but for now this issue is solved for me.