This is from the offical Google big thread found here: https://issuetracker.google.com/issues/68450197
After changing the APN Type string from 'default, supl' to <blank> for the internet APN in mobile network settings tethering unexpectedly works again. Not clear how these settings got there (part of the 8.1.0 upgrade?), but it works for now. Just for the record: This was on Australia's Telstra network.