This is new:
I saw this earlier today in Firefox (current version) on Win 7, and I'm seeing it now in Chrome 61.0.3163.100 on Mac Sierra. It only happens on chat info pages, not on Q&A sites. I suspect there's a Z-order problem with those time labels on the chat pages.
The numbers are sticky by position; if I scroll the achievements pane those times overlap whatever is in that place in the pane.