Hoya yuennanensis was first collected in 1915, but was not published until 1936. In 1977 botanists Tsiang & Li believed that H. yuennanensis was a synonym of H. lyi, but Michele Rodda in 2012 said that they are clearly two separate species:

“It is clear from the examination of the lectotypes and of the cited specimens of

Hoya lyi and H. yuennanensis that there are two separate species, both ecologically

and morphologically. Hoya lyi is a small lithophytic species with variable leaves that

can be oval, elliptic or oblong. It is found only on limestone, in deep shade, growing

tightly appressed to the rock surface. Hoya yuennanensis, in contrast, is a more

rampant climber, generally with elliptic or oblanceolate leaves found on siliceous and

limestone outcrops in exposed locations. Both species have white to pale pink

flowers of similar size but they can be easily discriminated by the shape of their

coronas. Hoya lyi has a rather flat-topped corona with rounded and flattened

slightly ascending outer corona lobes while H. yuennanensis has erect corona lobes

with rounded outer processes and a depressed stigmatic head (Fig. 1). They can also

be discriminated in their pollinia: Hoya lyi has smaller pollen masses with an

elongated retinaculum while H. yuennanensis has a massive retinaculum compared to

the size of the pollinium (Fig. 2). For these reasons Hoya yuennanensis should be

reinstated.”

https://www.researchgate.net/publication/259418725_Taxonomy_of_Hoya_lyi_Hoya_yuennanensis_and_Hoya_mekongensis_Apocynaceae_-_Asclepiadoideae