Water whose source is on a stair doesn't flow into stairs beside it

Water doesn't flow through waterlogged blocks.

From this changelog:

Water can flow out these blocks, but can not flow into them

I admit this, but isn't that intended to be changed in the future? In other words I believe this is just an acknowledged bug, and not a feature working as intended.

For now, this is intended. As @unknown explained here, this is to prevent contraptions from being flooded.

OK. But it's kind of a contradiction that the waterlogged stair in the first screenshot doesn't make water flow out onto grass, but the waterlogged stairs in the second screenshot do (and only to the back of the stair, not the front of it). Why's that working?

That seems to be a separate issue. The water isn't spreading because it thinks it can flow down the nearby blocks but the stair is blocking it. That's now MC-129030.