LTspice (like all other SPICE3 compatible simulators) does a .AC analysis by replacing all nonlinear components with linear equivalent models calculated at the DC operating point, to derive a linear equivalent circuit that can be solved in the frequency domain. Saturation, clipping and other non-linear effects are not modelled during .AC analyses.
Therefore the only effect of setting non-unit amplitude for a current or voltage source used as the signal input is to shift the reference level of the resulting plot. e.g. set AC 10 and 20dB will be added to all amplitude result curves, set AC 0.1 and 20dB will be subtracted, etc. Its therefore usual to simply use AC 1 for the source or sources that are inputs and leave it blank for all other sources.
Your source with AC 50m simply subtracts 26dB across the whole frequency range, as 20*log(50mV/1V)=-26dB, with no other effect (except confusing you)!