Instrumentation amplifier output attenuation in LTSpice
Your amplifier is saturating. You can't expect sensible results from the AC analysis if the amplifier is not behaving more-or-less linearly.
Doing a transient analysis before the AC analysis will make this obvious.
Your expected DC gain is almost 600, so about 20mV at the input will saturate the output with zero input. Calculate the DC offset from your 270K resistors and the typical input bias current and you will have your answer.
In a real design for a serious application you would have to consider the worst-case Ib which is about 6x worse than the typical.