It doesn't happen always. In the attached file the horizontal dimension is removed, which also is not what is expected. Because I was on the wrong track as well I give a recipe to reproduce:
- open Sketch in Sketcher
- check Auto remove redundants
- uncheck Auto upadate
- select arc and line - not any points!
- select tangent icon
Expected is a fully constrained sketch with a horizontal continuation of the arc. The length of the continuation should be 50.