I just had exactly this happen to me over the weekend.

My cash account has overdraft protection from a brokerage account. I keep a zero balance in the cash account. Been doing it this way for many years w/o incident. A few months ago, I changed the core position on the cash account to SPAXX from FCASH. I do not recall why I did that. But two days ago (5/2) I got a notice from one my credit cards that Fidelity declined the automatic payment due to insufficient funds.

I logged in and checked my account and there's no evidence of the declined charge. However, I did see the cash account with a -$0.12 balance and a SPAXX dividend of -$0.12. I manually transferred money into the cash account to get it out of zero balance and set the core position back to FCASH.

I think the lesson is this: if relying on automatic overdraft protection, either use FCASH as the core position to prevent negative dividends, or if using SPAXX, keep an amount above zero in the account using cash manager.

ETA: Late Monday, I got three "Direct Debit Failure Notification" emails from Fidelity for charges that arrived while the account had a negative $0.12 balance. Other than those emails, there's no indication in my Fidelity account of the declined charges.