It would be nice, but it's not a trivial fix for such a minor issue. In fact, looking even further back this was marked 'As designed' so I'm not positive we'll wind up making any changes here:
This is generally as designed. As noted, if no year is selected, we don't have a date format that can handle that. It's not trivial to try to convert the full date format to one that doesn't have the year, so we pick one (that we internally refer to as monthDay).
If this is significant to you, you might need to customize the member_about template to override the birthday display format.