How old is that soil? Was the plant over watered at any point recently?

I’ve probably had this Hoya 6 months or so and I’ve never changed the soil, so I have no idea. It hasn’t been overwatered in my care haha! Probably the opposite but possibly before I purchased.

This happens to random Hoya of mine. Sometimes it happens on thick leaved Hoya until they firm up (see picture, H. marginata) The whole backside of a young leaf will get black spots. They eventually go away. Other Hoya get little 'bug bites' that stay forever. I don't have any infestations of hoya loving pests but I imagine some random bug decides to try a bit then moves on. I have also thought it might be related to quick temperature changes. In any case they don't seem to hurt the plant other than superficially. H. pubicalyx "Black Dragon" has had these spots for over 3 years. (see smaller leafed hoya pic) It hasn't gotten worse in that time.

I hope this helps.