....that seems to be normal then I suppose...
Yes, that is normal.
The Delta NG gauge on the B2, is a ''somewhat early model'' of the FLI on the B3.(Only for NG of course) as it will depict your Altitude-Temperature corrected Max T/O power setting when the needle is pointing to 0 when NG is your limit.
So, the ''dropping to zero'' you're referring to is in reality below the -10 mark on the gauge and not 0 which is right at the top.