I would never have thought to try this because from my (admittedly basic) understanding is that Portainer and Kubernetes are both orchestrators at a basic level, but architecturally quite different in their approach (and capability).
Portainer seems to be good at running a "stack" of containers on a single system (ignoring infra-as-code/edge deployment for this arguement).
UMH on k3s is designed for a much more distributed deployment including replicas / load leveling, etc., even though you CAN run it on a single system.
To run one inside of the other wouldn't even come to my mind...
@Jermuk is this a valid use case?