Ordinarily it would be x yappianum for a natural hybrid if it is recognized as such, but only for a plant from a natural population. If the same hybrid is later made artificially it would normally be Yappiaum.
In this case, yappianum was put forth as a species name for a plant that may have been an artifical hybrid. To be x yappianum it would have to be a real natural hybrid, and I think it would need a valid publication as such, so x yappianum may not be a valid combination. RHS has it as yappianum, though they call it a natural hybrid.
Meanwhile, Jogjae was registered for the artifiial hybrid. Assuming there was not a valid publication of x yappianum for a real natural hybrid before Jogjae was registered in 1927 there is no problem with that. Oddly, RHS lists yappianum as being a synonym, but says Jogjae is not a synonym.