Definitely don't fret too much over humidity. So long as you aren't drenching the walls of the enclosure and increasing the humidity intentionally, any natural fluctuation in that range is fine. I live in a moderately humid climate so, especially if that's the case where you live, it will happen.
When I see Mizu flop down on his belly and lay his legs down at his sides, it reminds me of a kid falling asleep on their belly during a nap. I often get comfy in bed on my stomach myself (regardless of what people say about sleeping on the belly, it just is comfortable to me), so appreciate when they're relaxed while basking or when they go to sleep. Means they're in a good place.