Dental rescinded after cleaning?

Good morning, i recently turned 26 and had a cleaning done because i received a letter before my birthday that my benefits would cease at the start of July, i went in around may to get a basic cleaning, then received a retroactively dated letter saying that my insurance was cancelled prior to my appointment date now instead of July, but it was sent after my appointment already happened? Now they’re refusing to pay saying I owe $300. Is it worth fighting the insurance company? I have the letter originally first dated before the appointment that says i have coverage until July.