When there are (positive) changes in your reputation, this is reflected by the "+x" in a green badge on the trophy icon. When you open the achievements dialog and click somewhere else to dismiss it, the "+x" briefly changes to "0" before disappearing altogether. It used to just disappear immediately, but since a few days or weeks I'm seeing that zero (on multiple devices and browsers).
I'm not sure if it always happens; here is a screen capture:
