While I totally see our point and can understand the outrage, at the same time:
1. The Z3 was known to also have overheating issues
2. The Z5 uses an 810 which is renowned for overheating
3. 4K is an incredibly taxing job for even professional cameras, let alone tiny little mobile phone cameras
I think you're expecting too much if you want this relatively tiny little device to be able to record 4K video at 60fps with absolutely no issues whatsoever. The way I see it, you have to understand when something is basically marketing bull just to say they have it, and when something is an actual practical feature. Just like the way they have the "world's first 4K mobile phone screen!©®™", which only activates under certain conditions, or by saying it's waterproof, but only under certain conditions (fresh water for 30 minutes).
Plus this is hardly the only phone to have limitations or also overheat when recording 4k (as mentioned by others, G4 had issues and Samsungs have a time limit when recording 4k).