This hoya species is still rather rare and it has been going with the name "Hoya sp. flat stem" since its stem is flat, not round.
When I got the plants, they were mistakenly named H. multiflora for its up right stem and elongalated leaf form.
Now I'm excitedly waiting for the first buds to open to see how it is different from multiflora! Many thanks to the friend, who sent us the plants last year.