I am baking at high altitude.
The middle of the buckle did not look undercooked when we cut it last night. We did not refrigerate it overnight. It looks like the middle is not cooked, but we cooked it for the full amount of time.
The simple answer is that the pan is too wide. The time it takes for heat to travel through and through to the middle of a shallow baking vessel filled with "watery" "liquidy" and/or very moist ingredients, causes sinkage in the middle. Often times crisps and buckles and such need to be "overcooked" on the outside rim, before the interior is fully baked.
Look at your pan and think of it as if it were a doughnut. The hole takes a really long time to bake through.
Overnight, moisture that did not evaporate, keeps leeching into dough. A lot of bakers will macerate and drain their fruit before making this family of desserts. They want as little moisture there as possible.Of course it depends what kind of ratio you like. I like more fruit than crust or topping, so I am not willing to sacrifice any of the fruit aspect. But sometimes I will macerate my fruit, drain the liquid, and reduce it on the stove until it's a little "jammy." Then I will add the fruit into this loose jammy liquid, and mix in a few pinches of flour, to ensure that it fully sets.
Blueberries are orbs filled with juice. Depending on their size - bigger blueberries have a higher moisture content than small/wild, they will always continue to release moisture after baking. This is why a blueberry muffin has to be eaten the same day, but a raisin muffin does not. The dough surrounding fresh fruit in a muffin will always look raw. Especially if the blueberries were frozen going into the batter.
Does this help? O how I love cobblers and slumps and buckles and crisps! Summer is The Best season for these lovelies.