This was a side-effect of the canvas renderer ignoring the renderer-specific alpha multiplier. We use this to ignore the alpha of an object when we call bitmapData.draw, but it only behaved this way when using native targets, not HTML5, due to this overlooked mistake in the canvas renderer.
It was represented in the changelog as “Improved the rendering of cacheAsBitmap objects with alpha” as it fixed some bugs in cacheAsBitmap rendering.
We effectively perform a render with a scale applied. How a scaled down render is accomplished is a little different depending on the renderer. Does the smoothing property make a difference?