Solutions
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thebluespirit's post in two-disk cache pool won't mount. Unmountable: wrong or no filesystem after array stop and start. was marked as the answerI was finally able to get the new ZFS pool to start properly. There was a file that existed in /mnt/cache which was preventing the directory from being removed which was interrupting the formatting process. After removing the file I formatted the cache pool drives again and they were able to be mounted.
I still don't know what happened to the original pool, but as I mentioned half of it was very old and half was very recently added. Not sure why the new drive wasn't partitioned by UnRAID, but that was a hint that something wasn't quite right.
Thanks for your help @JorgeB