Bode plot of buck converter incorrect?
I have captured your circuit using the PWM switch model in voltage mode and its equivalent differently derived by the CoPEC authors. The circuit is shown below:

First you need to check the dc points and confirm that they are correct and similar between the models. Both circuits deliver 270-271 V which is ok for a 50% duty ratio. We can now ac sweep both circuits and check their frequency responses:

The control-to-output transfer functions are rigorously identical which confirms that models are ok. I prefer symbols where you see the internal arrangements, it is easier to check for mistakes later on in my opinion.
If I now apply the DCM formula that I have derived in my next book on small-signal modeling, the Mathcad sheet delivers a response similar to that of SPICE:

As you may know, I am releasing a new book on small-signal modeling of switching converters (TOC is here) in which I derived many transfer functions. The one for the DCM buck operated in VM was originally derived here.