kexec not working on C1 ARM server

[anisse]

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.