LTSpice transient analysis and current amplification problem
what am I missing?


This: -

In an AC simulation, 0 dB is the default reference value for the voltage source and that strictly speaking means 0 dBV or 1 volt RMS. So, when your tuned circuit is fed precisely the frequency that resonates it, all of that 1 volt RMS becomes applied to your 10 ohm resistor. That generates a current of 0.1 amps RMS or, put another way -20 dBI or loosely put, in terms of the vertical scale on your graph you have it peak at -20 dB.


  if the attenuation is -20 dB....


It isn't; the current through the 10 ohm resistor is -20 dBI or 100 mA RMS (as one would expect when 1 volt RMS is applied through a resonant series tuned circuit).