I have exactly the same problem with my site. My customized User Settings are not retained, and it is causing me grief.
However, I can replicate this effect EVERY time. For me, what is causing it is the Forum module. Any time a user views the Member List or updates their personal settings, all the core User Settings for the portal revert back to defaults.
The developers in the Forum Module have explained very convincingly that it is not a bug in the forum module -- it sounds like something in the core. Hopefully it can be addressed.
Oh, FWIW, I am running DNN 4.7.0 and version 4.4.3 of the Forum Module. So I don't think rolling back from 4.8.0 to 4.7.0 is going to help you.