Not quite correct, in older kernels it used to not let trigger sleep modes when serials were enabled per the UART clocks, really recent kernels have some bright UART clock management that sleep when serial inactivity is detected so full PM applies.

The kexec hang problem have to do with initialization delay timings, when serial-console is enabled in the R&D flags the new kernel will boot slightily slower but it does the trick and works, without it won't boot just hang.

FYI I was able to kexec a kernel without serial-console flag when I insert printk delays in the kexeced kernel which could be useful to discover what's wrong but I didn't progress further, obviously this is not a solution as one will have to replace distro kernels with kernels with delay mods which is _wrong_.