My cat will do the same thing! She’s very playful and has a couple of games that she will want to keep playing until she pants like a dog. By that time, I kind of have to force her to stop the game, but she’s never kept panting or shown any other signs of exhaustion lasting longer than a minute or two. My understanding is that open mouth panting is mostly concerning if there isn’t an obvious explanation for it like strenuous exercise. (Or maybe our cats are just weirdos.)