Chosen Solution
I have a space gray iPhone 6 that recently just had a battery replacement done to it. I performed the replacement. The buttons worked before the battery replacement and after. They stopped working after I did a restore to iOS 12.1.3. I have tried disconnecting and reconnecting the connectors for these buttons. The home button and touch id still work. The battery works fine and in general everything else works fine.
It sounds like the volume / power flex was torn during the battery replacement. Did you insert a prying tool to remove the battery on the upper section of the battery?
