Prescription covered even though my insurance was only paid up through July— If I plan on switching insurance for August, can the previous insurer come after me for the cost?

In a bit of a sticky situation here insurance-wise. I’ve been on BCBS through COBRA since May due to being laid off. I’ve paid in monthly since then, up through the last day of July. Fortunately, I began a new job on 8/1. They offer insurance which I’ll be signing up for in the next week or so (I have 30 days to pick a plan and it backdates to 8/1). In the meantime, I made the dumb mistake of waiting until 8/2 to refill my BC pills. I fully expected to pay the retail price, then perhaps appeal my new insurance to reimburse me when I get it squared away.

Instead, I showed up to the pharmacy yesterday (Walgreens/Duane Reade) and the person said that my old insurance on file (BCBS) covered it. I tried to explain that technically I should only be covered by them for July, and are you sure I don’t have to pay the retail price? I’m not renewing and will be starting a different plan for August. For some reason, this seemed to confuse him, though I don’t know why it’s confusing— people use COBRA and have gaps in coverage all the time?? He said no, they covered it, and mumbled something about it being filled on 7/31. At the time, I accepted it because he was being quite pushy about it and the store was closing. He gave me a print out of a form that included a long string of numbers, saying I could reference that as proof of the transaction that BCBS approved. Later I look at the prescription info on the bag and all the indicators for the fulfillment date are listed as 8/2. Nothing about 7/31 like he mentioned.

After thinking on it, I just want to brace myself for a scenario where they come after me for the cost of the pills. Ultimately it’s not a massive amount— maybe $150 at most, but I’m dreading the idea that once I don’t renew for August, they slap me with some kind of delinquency without notifying me and put me in collections or something. Is there anything I can do to get ahead of it or fix this?