LTSpice produces incorrect simulation for Class AB amplifier
With base bias resistors of 10K, two series diodes have too-little current flowing through them. At LTSpice's 27 degree C default, only 0.5569114 volt drops across each diode. That's not enough to bias the complementary pair of transistors with much "class AB" DC bias current - only 23.66 uA DC is pulled from the 12V supply through Q1,Q2. This is more Class-B than Class-AB.

AC analysis proceeds:
Transistor characteristics are first found with 23.66 uA DC current flowing. All the transistor parameters are linearized at this bias point.  Base bias diodes are linearized similarly.
This is where simulation problems can arise...diodes become resistors, transistors can have unreasonably large (or small) current gain because these parameters are extrapolated linearly to extremes.

Transient analysis gives a more-true simulation, but requires much more involved and time-consuming calculations. A transient analysis, done with tiny input amplitude approaches similarity to AC analysis. Its results converge with the (bogus) AC analysis (left):
 When the transient analysis is done with higher amplitude (plot at right), you can see the cross-over "glitch" where the small-amplitude (plot at left) is done.
These two transient runs appear to disagree, yet each is a reasonable result, scanning different sections of a non-linear transfer-function.