I have had the same problem @Waggo but only a few times.
Cannot recall what game it was but the Series X turned off a couple of times some months back, getting the message on reboot its to do with overheating. No issues since until Flight Sim. It turns off every time I play it now after about 15 or so mins so had to uninstall sadly. However, my son has since played plenty of Fortnite and no issues.
Like you, plenty of ventilation, I’ve even moved from horizontal to vertical and plugged straight into wall. Even performed system ‘reboot’.
Im gonna keep an eye on it over the coming days but if it happens again, I’ll have to send off given there are only a few months left on the warranty (got mine at launch). Sorry to hear yours is still playing up. There are plenty of reports of the issue when you look online but nothing to the point where Microsoft have acknowledged it’s a problem.
I agree with you that the customer service could be better. Even just an email explaining what they have done with the repair would help. You would hope that was a brand new unit they sent you but what’s to say it was a refurb?
I owned an Xbox One X previously that I had in my conservatory and would get very hot in there in the Summer months. It ran perfectly. Dread to think if my Series X was in there now.