I get the following traces when running kexec :
[ 129.248360] kexec_core: Starting new kernel
[ 129.298586] Disabling non-boot CPUs ...
[ 129.393572] CPU1: shutdown
[ 129.532515] CPU2: shutdown
[ 129.632399] CPU3: shutdown
[ 129.700758] Bye!
No new kernel is being run it seems.
I have tried using the KEXEC_* server tags, and also running it manually from the initrd dropbear (detaching nbd devices first). Kernels are the default ones provided by Ubuntu 18.04.