iPhone untrusted certificate warning
Yes, it seems to be a MITM attack. The pcr.apple.com has a certificate issued by Symantec for Apple - but you're getting an entirely different, self-signed certificate. 

Note that MITM doesn't mean that it must be a "malicious" attack as such.

You do not mention the internet source you're connected to when the problem occurs. Some connections, particular public WiFi such as in restaurants, on trains and busses, etc. will use seemingly MITM for presenting sign-on pages for their WiFi, or for the purpose of injecting ads into web pages.

The certificate you're getting is from an Icomera system. They supply systems for passenger WiFi on trains and busses. If you're using such WiFi, this is the source of your problems.

And yes, it is a terrible practice for Icomera and others to be building their systems like this.