Are you able to post a packaged workbook (twbx file) or screenshot so that we can better understand the issue?
This does seem like a bug in the reference line tool tips.
I created a workaround using two Gantt charts. One for the reference bar (starts at SUM(LOWER) and the height / size is a new calculated field SUM(UPPER) - SUM(LOWER).
The second Gantt chart has the Measure Values for SUM(UPPER), SUM(LOWER) and SUM(MEDIAN). This creates three slim bars, but when the upper and lower are colored the same as the first Gantt reference area, they blend in.
This also has the advantage of providing a normal tooltip at each of the values (upper / lower / median). You can of course control the text that's in here.
ReferenceLineIssue_jimw.twbx.zip 36.2 KB