Parent Task Duration appears to not be calculating correct

[MichaelSMN]

We are working through a process of adjusting task duration down to 0 days in the event that a task is marked as not required, essentially streamlining our timeline if a user says certain tasks are not required.  When doing so, a weird thing appears to be happening where the parent task is not reflecting the correct 0 day duration and appears to be holding onto some days for some unknown reason.

Any idea why the parent task would show anything other than 0 days duration in this image (its holding onto 2 days somehow)?

On a second test project, i manually typed out a very similar layout with 1 parent task and 4 sub tasks, i set these all up with a duration >0, then changed them to 0 just to test the same process.   In this example, the parent task is calculating correct with 0 days duration.