I poured about 1/8 of the cider into a pot and added about 2 Tbsp cinnamon and 1 1/2 Tbsp of vanilla (complete guesses). I simmered for roughly 10 min, added it back to the main batch, gave it a good shake, and tossed it back into the fridge. The next day I came to find something somewhat unsettling...floaties. And not just a few little things here and there, but it appeared as though I had added a large amount of brown pond algae to the batch. But it couldn't be anything of that sort, and clearly THAT much mold wouldn't grow overnight, and especially not in an alcoholic environment!
I've managed to separate out the cider from the unknown substance, and when I did, it looked very similar to cinnamon roll icing, and absolutely stunk of cinnamon. I'm guessing the cinnamon I added absorbed some cider and formed a gel like substance? Either way, it's been disposed of. I've got the cider chilling in the fridge, and will give it a try shortly. Hopefully no floaties appear this time!