Expected and actual unity gain bandwidth LM324N

Gain-bandwidth product is a small signal specification. It only applies to signals under which the op-amp circuitry remains in a linear regime. That is on the order of 100mV or less for a conventional differential input stage.

With the relatively high frequency 2V you are applying you are well into a large signal regime in which non-linear effects take precedence. The specification that applies in that regime is the slew rate of the amplifier.

Slew rate limitations are caused by how fast the internal bias currents are able to charge the internal (compensation) capacitances.

The LM324 has a maximum output slew rate of 0.5V/\$\mu\$sec, so it not going to be able to accurately reproduce a 2V amplitude sine wave of more than about:

\$f_{SR} = \frac{0.5\cdot10^6}{2\cdot\pi\cdot2} \approx 40kHz \$

To see the unity gain-bandwith directly you can reduce your signal amplitude to perhaps 50mV.