Yes, it is.
Especially so if you are out in the sun.
4K60fps + EIS enabled is demanding of the phone internals and the camera sensor itself.
In sunlight, or during summer, when the ambient temps are already high - this will cause a lot more heat up.
The problem is not even that it heats up. The problem is that the camera app shuts down after 3-4 minutes of 4k 60 fps and you cannot use it anymore until it cools down, which can take a while, especially if you are outside in hot weather. And then you can shoot another 3-4 minutes of 4k 60 until it shuts down again, and so on and so forth. And even at normal, room temperatures, you cannot shoot more than 6-7 minutes of 4k 60 fps before the camera shuts down.
I understand your technical standpoint. However, from a consumer point of view, it doesn`t make sense to put feature in a phone if that feature is so severely limited, and even borderline unusable during summer holidays, when people are out and about, filming stuff. And this is too bad, since the quality itself of the 4k 60 footage is very good.
So, I really hope for all of us, fans of Asus, that a technical solution to a consumer`s problem could be found. In the end, every OEM knows full well, and in advance of launching new phones on the market, that the combo between a small phone without any special cooling system and 4k 60 fps is a bad one.
We're looking at improving this bit in the next update
Tbh this phone is warm no matter what you due.