Why did the apple skins turn blue/purple-ish in my apple pancakes the next day?

I feel fairly confident that what you see is anthocyanins (naturally present in apple skin) reacting with some leavening in your pancakes.

Here's a link that explains in more details, but I'll summarize: https://extension.psu.edu/fruit-color-promoting-red-color-development-in-apple

Anthocyanins are a natural pH indicator present in many fruits and vegetables. Red cabbage and blueberries are common examples. Elderberries and black sweet rice, and even many flowers also contain these compounds.

And they turn red in the presence of acid like vinegar, buttermilk, or fruit juice, and blue in the presence of a base like baking soda or baking powder.

So when you bake or cook with certain fruits that contain anthocyanins, you might be surprised that your blueberry muffins came out green, or that your "purple" rice is hot pink. This is because anthocyanins are water soluble, so they'll disperse throughout what you're cooking easily, and visually indicate the pH. You may even notice a blue shadow in the pancake around or under the apple skins.

But if this is the case you are a-okay. Judging by the contents of the recipe, it only contains a base for leavening, but no acid for it to react with. So it makes perfect sense that your apples, the only acidic component, would react and turn blue. If you want to get more rise, include some buttermilk or lemon juice next time. When it reacts with your leavening you'll get more floof, and your apples will stay red (or pink at least.)