I have only made an apple pie once. The recipe for the filling was just apples, sugar, and berries. And it cooked for ~45 minutes so the apples got soft and some of the juice from the apples dissolved all the sugar. It sounds like it may have been cooked too long, or maybe apples acted very different without sugar. I'm no expert though.
Me neither, unfortunately. Although I can hazard a guess chemistry-wise. I think that my failure was a combination of: - crust that wasn't porous enough to allow quick enough escape for water that was effectively 'soaking it in' ; - too high moisture to begin with; - using apples of the sour variety that changed pH to more acidic (I never really liked sweets); - possibly too high temperature; that as a result allowed much more glucose/fructose to caramelise and seal the lower part of crust. And since most of the water was removed from the top crust, it probably was unable to bake properly inside, while getting all that 'too much Millard reaction' tinge on me. I'm perhaps talking out of my arse, but so far my cooking was 90% chemistry and 10% wondering WTF went wrong. ;)I'm no expert though.