Loading a combined kernel, initramfs, and boot options as direct loadable efi stub

[NiKiZe]

You modify the kernel instead of using the kernel configuration option (CONFIG_CMDLINE) ?
Which exact kernel version/sources is this? (for anyone that would like to try and reproduce the issue)

In the case of iPXE you always get a cmdline input - my guess (yes purely guess) is that this overrides the cmdline that you add.
The only way to figure out what is going on here is probably to debug the cmdline handling in the linux kernel.

I can not find much information on what the .cmdline is refers to, but there seems to be several related options that you could use to change behavior here. And there is also several bugs with embedded cmdline handling, but all that of course depends on which version of the kernel is used.