Hello…….
According to what I could find, Hadaya Vatthu is formed by kamma in the patisandhi moment in the arupa loka together with Pasāda Rupa (C, S, G, J-Vatthus), where Pasāda Rupa do not necessarily have to arise with it. So Hadaya Vatthu itself is considered the lowest rupa and they did not bother to call it Hadaya Vatthurupa because in the word Vatthu, the Vatthurupa is included. Because if Hadaya Vatthu did not have the connection to Vatthurupa, the statement that the world consists of 28 types of rupa would not be correct.
Hadaya Vatthu is therefore considered the absolute basis as a fundamental rupa for Nama+Rupa in all 31 realms………..
Sādhu, Sādhu, Sādhu….