Talked to Apple support. I had to select a different language and then everything worked. You can use the vocabulary function with American English, but it doesn't appear to be available for any other Englishes. I tried adding special spelling to the vocabulary option, and initial tests in Microsoft Word suggested that it worked. However, it didn't work in Scrivener entirely and when I tested it in Word again, it reverted to American spelling.
I write for Canadian audiences, so not having the proper spelling could be an issue, but I use software to double check my spelling, so hopefully I can figure out a workaround. At the very least, dictation doesn't randomly stop anymore.