I'm trying to make retina screenshots (144ppi) into a normal size (72ppi). In the past, I've just copied and pasted them into a 72ppi document and manually resized them 50%. However, I was told by some co-workers that this wasn't accurate enough. Instead I was directed by a guy to change the resolution in the Image > Image Size dialog box. However, when we did this, we found my image dimensions wouldn't change like his did even though all Preferences seemed to be the same as his. I've tried restarting Photoshop CC and my computer. No dice.
There's no bug. A screenshot is just so many pixels wide by so many pixels high. It doesn't have size, it doesn't have ppi. Those terms don't apply.
The ppi setting is a print parameter. It defines the pixel grid on paper, thereby defining how densely (how big) the pixels print out on paper.
On screen ppi doesn't apply because there already is a pixel grid - the native resolution of the screen. So ppi is defined by the screen.
IOW your screenshot will display at different sizes depending on the screen it's displayed on. It falls into the already existing screen pixel grid. On a retina screen it will be small, on a traditional screen it will be bigger.
Resampling (changing the number of pixels by interpolating) will work, but inevitably blurs the image because pixels have to be invented or thrown out.
I was going to post an addition to the above, when we all suddenly got signed out...
What's special about retina screens is that the UI needs to be upscaled on-screen to be readable, because with the high pixel density it would otherwise be too small. So an original pixel in the UI is represented by four pixels on screen. This applies only to the UI, not the image (that would defeat the whole purpose of the high resolution screen).