Rendering artefact from calculated column

Today I noticed yet another curious quirk both on the dektop and on the service.  It seems to affect chart visuals as well as table visuals, so it really seems something's off with the data.

The problem is that the X axis comes from a rather innocent calculated column

Close M = format('Facts'[Close Date],"MMM-YY")

In the raw data view there is no discernible difference, no leading space, no trailing space, nothing.  Has anyone seen this behavior before and maybe can you suggest a fix, or at least where to check?