I see that the cursor looks like two arrows ↔ which means that you’re trying to resize the task (change its start or end date and change the duration). This makes no real sense for milestones, but apparently when you start this interaction closer to the left corner of the diamond, it works and really changes the milestone start date, but keeps its duration as zero. Effectively, this is “move backwards”.
For move interaction you need to position the mouse pointer over the diamond center, so that it looks as a hand (or something similar on other platforms)
See the screencast below showing these resize and move interactions applied to the regular tasks and milestones.
