I tried your file and unfortunately (or fortunately) I still can't replicate the issue on 0.15.2. It only occurs for me on 0.16.
I really want to help you get to the bottom of this so I added another video comparing the two versions and how it only occurs for me on 0.16. You can see on version 0.16 when I rotate the object on the X axis with the gizmo, if I rotate the object more than about 48 degrees (watch the values in the transform box on the left) it just snaps straight to 90 degrees, and this is shown when I change keyframes. You can see what I mean in the video.
I am curious if you are able to replicate this process on both versions to see if the same occurs for you? If it occurs for you on 0.15.2 I have to wonder if it is settings related.