Once a file has been saved without a merged compatibility layer (via the file-save option Maximize PSD And PSB File Compatibility = Never), File > Save will never re-save the file with a compatibility layer, regardless of the setting of Maximize Compatibility. The only way to add the compatibility layer is to ensure Maximize Compatibility = Always and then do File > Save As to save a copy.
One noticeable impact of this is that people trying to get OS X Quick Look to display thumbnails and previews by changing Maximize Compatibility could get confused and believe that the option is having no effect.
To reproduce the bug:
1. Start Photoshop CC 2014 on OS X 10.9.5.
2. In Photoshop > Preferences > File Handling, set Maximize PSD And PSB File Compatibility to Never.
3. Restart Photoshop.
4. Create a new RGB 8-bit document with two layers and visible edits on each layer (e.g. with the pencil tool).
5. Save the document as "test.psd".
6. In the Finder, view the folder containing "test.psd", select "test.psd", and do File > Quick Look. Observe that the preview is all white, indicating that the merged compatibility image has not been saved (as expected).
7. Now change Maximize PSD And PSB File Compatibility to Always, and restart Photoshop.
8. Make another edit to one of the layers.
9. Do File > Save.
10. In the Finder, do File > Quick Look and observe that the preview is still all white (not expected -- BUG).
11. In a Terminal window, give the following command to flush the Quick Look and Finder caching and restart Quick Look and the Finder:qlmanage -r; qlmanage -r cache; killall FinderObserve the preview is still white.
12. In Photoshop, do File > Save As to save a copy of the file as "test copy.psd".
13. In the Finder, observe that File > Quick Look properly displays a preview of "test copy.psd".
I have verified that when Quick Look is displaying "test.psd" as all white, the Boolean "hasRealMergedData" in image-resource block 1057 is 0, and the merged compatibility layer consists of all white pixels. This indicates that Photoshop hasn't saved an actual merged compatibility layer.