Those roots look perfectly healthy. To me, diptera and vanuatuensis are nearly identical (flowers are ever so slightly different...) and they both have a tendency to react this way to both over and underwatering. I lose several leaves every year to leaf drop when my watering isn't spot-on. It's just a little sensitive, so be extra careful with it. It seems to do better when I err on the side of dry, but it's a fine line. A south window, I think, is a little strong for most Hoyas, and this being one. It'll be happier in a west window.

But I will also add that I've tried the water meters, and I think using the "pokey" method is far safer. I got a meter once, to see if maybe it was a better predictor than I am, and I would use the meter, it would say to water, then I would "feel" my soil and say, uh-uh, not ready! Maybe there are ones that are more sensitive to moisture than others, but the one I had said dry long before I would have typically watered. They might be great for tropicals that need more constant moisture, but I wouldn't use one on my Hoyas...

Denise in Omaha

Thank you Denise. Great point on those meters and Hoyas. I may just use it for my citrus trees.

Cut back on the ferts for a while and see what happens.Also a flush with water when it is dry will help get any extra fertilizer salts that have built up.I clean my meter every time i water with a green scouring pad. The copper can oxidize in only a few days and through off the meter.The meter should peg the wet side when placed in pure water,if not you need to polish it more with the pad.

Jimmy, thanks for the tip. I always wipe the meter with a damp paper towel between measuring each plant. I never thought to scour it but now that you mention it oxidized copper could have an effect on the reading. Thanks.