After updating to the 1.11.4 firmware, it's not possible to boot from a custom kernel that worked under 1.11.0. This is with udm-kernel-tools 1.1.2.
root@ubnt:/# udm-bootctl boot 4.19.152-edge3
Loading kernel 4.19.152-edge3...
Inserting kexec kernel modules...
Kernel version not supported (v1.11.4)
Make sure you have installed the latest version of udm-kernel-tools...
After updating to the 1.11.4 firmware, it's not possible to boot from a custom kernel that worked under 1.11.0. This is with udm-kernel-tools 1.1.2.