I lost my tether after the 4.4.2 update as well. I have a Sprint Nexus 5. My laptop and tablet were able to connect to the hotspot, but no data was coming across.
I looked at the settings.db file and could still see the "tether_dun_required" entry was definitely there. So I decided to replace the settings.db file with a copy I had from before the 4.4.2 update. Rebooted the phone, and magically, tether started working again.
I was curious though to see if I put the post-4.4.2 update settings.db file back, then would I lose my tether? So, once again, I replaced the settings.db file with the post-4.4.2 version, rebooted the phone, and magically, tether still was working! WTF!
The only thing interesting that I noticed during all of this is that every time I replaced the settings.db file on the phone, the corresponding settings.db-journal file would disappear. I don't know much about SQLite, but I wonder if the journal file had some transaction state with the "tether_dun_required" hack removed or not committed.
Anyway, hope this helps.