As @otso pointed out I have quite some experience regarding RuuviTags in Sauna. I’ve had a tag in my Sauna for over 3 years now, without enclosure, with typically one Sauna session a week, and no signs of problems yet.
I have written about these observations a few times before, here is a sample of my comparison with and without an enclosure, and a few words about “tag safety in sauna” on the next post right below the linked one. And here I wrote a few points about some of my observations regarding Sauna and RuuviTags