Problems using the "Must Start on" field using the RTC scheduler

I create a Work Item task and attempt to use the "Must Start on" constraint.  When I set the constraint to a specific date, say April 25, 2018, the ghant chart shows the actual date is May 6, 2018.  After some investigation, I found that there is an aprior task that is scheduled to be completed May 6, 2018.  The constraint type is acting more like a "As Soon As possible" constraint type and ignoring the "Must Start On" constraint.  How do I fix this.

Note in the above example, I have a single resource working 100% on the project.  

Also note that my project is not Agile... instead it is FPM.