Yeah, the same thing happened in the MX-5 - especially in high heat and/or when moving slowly, it stopped cooling and only blew warm air. Needed a refill. Leak checks on the system didn't find anything either, so in my case it has apparently never been serviced, or at least not for many years. Sometimes chain workshops like Euromaster and ATU have cheap offers on A/C gas refills / service, I've used that to get it refilled for like 50€ iirc, and it held up fine until I sold the car - may be worth looking into.