With the attached UI file check out the Tx 5 and Tx 6 tooltips in the preview in QTCreator.  Similar behavior is seen in the final app too.

Hovering over Tx 5 and moving to Tx 6 or to the slider bar widget and back to Tx 5/6 gives inconsitent behavior on each path you follow.  Sometimes the text is cutoff and sometimes it isn't.

Doesn't matter if you use rich text or plain text for the tooltips the display of them is very inconsistent.