I have been trying to look up any references to problems booting extended partitions, or possible special naming needed for it but with no success really.
There were some mentioning that the partition order would matter, that in some cases the partition naming would not be consistent.

I still think the problem is mainly this: When in ubiboot environment, whether it be menu or console, your partition is accessible when you mount it as otherwice the kexec_load would not succeed, not finding the kernel.
However, when you boot it the new bootin kernel cannot find it, due to some namespace conflict, maybe ?

A easy case to solve if you only had serial console on your device