You are correct. I noticed this issue.

I've searched and tested more about this strange behaviour and i confirm that :

- Capacitive keys works fine when phone is in hand.
- Capacitive keys are less responsive when phone isn't in hand (ex : put on table).
- Capacitive keys works fine when phone is plugged in charge.

It's a specific problem of capacitive keys only because when using an alternative app (soft keys replacement on main screen) it's OK.

Google search about this issue show that problem affect some of other phones and seems it's electrical grounding trouble ... (if body become like a ground contact when fingers are touching edges of the phone, it's OK)
Maybe Blackview could have a look and fix this specific issue with increase more sensibility for capacitive keys only ? (Main screen is OK in all cases for me)

Also, the new firmware seemed to help during the first few days. Now, the buttons are as irresponsive as they were before... Very strange! Anyone else noticing this as well?

Yes, first impression, it's normal because after new version update the phone battery is full charged, cache is clean because wipes, phone often is hold in hands, + placebo effect ... I've seen also that the keys are less responsive when battery go down. (test when phone is charging = no problem)
I think this problem is material/electrical for a big part and for me the best solution is to use an app "button menu (no root)" with more possibilities than BV6000 original capacitive keys ... more, with this app i get back a menu button for some old apps without 3 dots menu !