I think one needs to check the roots. Yellowing leaves can be from overwatering or some other drainage problem (like maybe the rootball has hardened off leaving the plant unable to take up water?).
A long time mentor of mine in plants used to tell our Indoor Gardening Club that trying to diagnose a plant w/out unpotting to check its roots is like trying to see through a blindfold, not likely to happen.
From what I've seen w/ some Hoya growers in Calif., Hoyas getting too much sun get a bit faded, bleached out looking (but still feel firm to the touch) & the leaves don't drop off.
May I suggest you take some cuttings off the green growth & pot them up separately in new mix just in case?
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