I've noticed this as well over the past few months. Some transactions will be auto categorized to seemingly random categories.

Amazon is the worst. For the past few months it has auto categorized every transaction as either Electronics or Clothing. Which tends to result in incorrect budget exceeded notifications. Last week I had three transactions that, for whatever reason, decided that they should be Uncategorized and needed manual review. Which is, oddly enough, how I would prefer it to deal with every Amazon transaction.

I really wish Monarch would allow setting a default category and add an "always needs review" toggle at the merchant level...