I had a transaction mis-matched.
This was originally (in the series) an 'exact amount' transaction, but I had edited the individual reminder because the monthly amount can vary with the budget. The transaction that was matched was no where near the exact amount (the transaction was less than $20, the reminder was over $800).
Does editing an individual reminder alter the exact amount setting?
I ask because if I need to I will instead create a series of "one time payments" instead of a series with edited reminders.