..or too much water!!!

I have only seen this happen with one Hoya in my collection and I think it may have been too much water as well. Have you been keeping this plant moist?
Remember Hoyas with thick succulent leaves usually like to dry a little before being watered again. Sub burn would show up as irregular patches that eventually take on a bleached papery look.

Mike

Another suggestion might be from over fertilization and the plant has some salt buildup? If that might be a possibility in your situation I would flush out the soil the next time it needs water.

Here is a link that might be useful: Short article explains some of the possibilities of brown tips

I was thinking along the same lines as Pug.

Browned leaf edges and leaf tips are very often the result of accumulated salts from water/fertilizer.
Heavy soils (such as peat-based soils) can exacerbate this condition. Also, ingredients such as coir
that haven't been properly rinsed can quickly lead to high levels of potassium, et cetera.

Josh

Good point Pug and Josh.

Mike