Your plant, to me, looks generally unhealthy -- discoloring, puckered leaves like it's not drinking properly, etc. It makes me think there is a soil issue -- too wet, or too rich, or a nutrient imbalance. Is that the nursery pot and soil it came in? That's probably the problem -- nursery soil is usually way too dense for hoyas outside of a controlled greenhouse. I would get it out and repotted NOW.
My Bella lives in low humidity too, same as yours or even lower, with a pebble tray under it that I am always forgetting to fill up. It's potted in a mix of maybe half regular potting soil and half orchid mix and pumice (this is a pretty typical mix for hoyas). I let it dry out fairly well between waterings, same as I do my compacta, and it hasn't dropped a single leaf in the 6 months I've had it. It IS very easily stressed and has dropped several peduncles though (particularly after my cat messed with it) but otherwise has grown like a weed and is already replacing the spurs it dropped.
Hope that helps. Good luck!
(Oh, P.S. -- humidity trays only create a little humid zone directly around/above them; they do not raise the overall household humidity. If you want to do that, then you need an actual humidifier.)