Photoshop: Resizing unlocked layers creates partially transparent pixels on edges

That's not a bug, it's a consequence of having transparent data and using interpolation. When interpolating the new values for pixels, the values of neighboring pixels contribute -- including the transparency values. And, depending on the resampling, you can end up with partial pixel coverage that needs antialiased edges.

Unfortunately, there is no good solution to the issue. It sounds simple when you think about a rectangle, but not so much when you think about a layer that has a flower arrangement with transparency.

If you know you don't want antialiased edges, you can change the resampling method to Nearest Neighbor.

I completly understand what you mean, but I think creating rectangle images (be it sprites or website GUI elements) is so important and common that in my opinion there should be an option (it could be a tick box within Resize Image dialog window) to keep the edges solid.
I can see there is no problem with the edges when the image is "Flattened" - so maybe this tick box could prevent the same behaviour?

Unfortunately Nearest Neighbour doesn't solve the problem as it affects the image appearance.

And am I wrong that in previous version of Photoshop the resizing engine was diferent as I can't renember the images getting half-transparent frame while scaling down?

I've noticed this too, but I am puzzled to as why it happens in a completely rectangle image with no transparency.

Take a look at this image:

It is literally an exact white square on top of an exact black square, yet when I resize I get 1px transparent border? Surely that can't be right, I definitely don't believe that this behaviour has been around forever.

Good rule of thumb, if you can't see it at 100% then it's not there and is most likely an issue of interpolation. There are a lot of small things like that.

This means a lot of extra work when creating art assets for iOS devices. Instead of being able to resize images to half their size for older devices, I have to remove all blending options (strokes and shadows), resize to half size, add new blending options at half the value of the larger size. This will create hours of extra work for our current project.