Can you just edit the Tesco APN with the 48 settings or do the settings also get reset?
Sounds like a problem with the phone's software.
Is the phone up to date with the latest software update from Samsung?
If so and the problem persists then I think you need to contact Samsung or the seller of the phone.
Hope this helps.
Since the issue arose on my old LG and one the new S8, I think the issue must be with the network software. Maybe something like the routine sending out of permissions to mobiles sharing the network.
As you suggested earlier, maybe Tesco Mobile still have my mobile number on some list - even though it's several years since I was a customer of Tesco Mobile.
Thanks again for your interest.
I'm not really sure why 48 support can't explain this if it happened with two different makes/models of phones.
I doubt that the network is pushing any settings out other than if requested via the Tweakker site.
Certainly none of the many phones that I have ever used or set up on 48 have received settings (even from the Tweakker site which has never worked for me personally!).
Sorry I can't really be of any more help here - it's annoying. :-(