TalkTalk router access blocked

This is an odd one.
A friend on TalkTalk reported that he was unable to connect to the exchange and that a TalkTalk engineer would charge £50 if the fault was at his end.
Since he hadn't a spare router to try I loaned him a DSL-2680 which had Plusnet firmware including TR-069 capability.
It was set up with my Plusnet login details but, if he got as far as connecting to the exchange he could change those.
However on connecting to the phone line the router automatically set itself up with the TalkTalk username and password and worked without any input - result a happy friend and he ordered a new router from TalkTalk (free with a 12 month tie in) and eventually returned my router.
Out of interest I tried logging in to the router using the normal admin/admin username/password and it wouldn't let me login.
The only way round it was a hard reset so that I could continue to use it as a spare.
Is this normal that having auto connected to TalkTalk it then blocks access using the default username/password

That is the point of TR-069

Since when - I understand it auto connecting although the Plusnet implementation doesn't as it checks the router serial number and "greenlight" PPP details which have to match the end user account - but I have never heard of it changing the router login details. That is the login access to the router not login to the internet

on changing the admin - not seen it before, but then not many people play around with tr069

Perhaps the TT remote management sets the router's own authentication credentials to TT's defaults rather than the manufacturer's ones.

Maybe this has something to do with it:

In reference to en.wikipedia.org/wiki/TR069#Security_and_authentication:

The identity of the device is verified based on a shared secret (password) at the HTTP level.