The subject line is pretty descriptive. Some of my hoyas are getting very many pin-prick sized black dots on the undersides of some leaves.

This is Hoya ridleyi. Old leaf is on the left (not my growing), new one is on the right.

I noticed the black dots, while present on the whole backside of the leaf, here form a thick horizontal stripe on the bottom half of the leaf. No idea why that may be. Any guesses?

Tops of the leaves look fine to me:

The new leaf is much more flexible, but that is entirely normal till it matures and becomes more turgid, I understand.

Here is a backside of a Hoya pottsii leaf:

Black dots ate not as dense but still present. This hoya is doing it on both old and new leaves.

Hoya micrantha is doing it too. Black dots are particularly noticeable on new leaves, but some old ones also have them to very small degree. No picture, but you get the idea.

And there are other species, too, where it's less prominent but present.

What should I read into this?