When delivering to your home the mailperson is not required to pick up mail from your mailbox.
My wife went in to complain at our local Post Office that our mailman did not pick up mail to go out, even though the red “flag” was up on our mailbox. “not required” was the answer by the PO counter employee.

…while that employee is performing normal delivery and collection duties. Letter carriers can pick up mail as they make their deliveries if the flag is up. According to the USPS click HERE Article Number 000002795.
Note the word “can” and not “will.”
Don’t place your Mail-In-Ballot in your mailbox and expect it to be picked up.
Turn your ballot in only to a staffed drop box that is emptied daily, a polling location or your county elections office