I might not assume it is a bug but rather a behavior that is inherent to fixed duration tasks. One thing to keep in mind is that the task duration is affected by the assignments. Example, you have two resources on a fixed duration task with the same work/utilization. One resource starts working on day 1 (actual start) but the next resource doesn’t start until day 3. In this case MS Project will attempt to extend the fixed duration task to account for the combined duration of each assignment (and consequently causes the warning message).
Could this explain what is happening to the task? I will tell you that MS Project works much better with fixed work or fixed units as the default task type. MS Project wants to flex the project for you as you manage the schedule and using fixed duration chokes MS Project from this natural function. It’s a long story but I digress…
Let me know if this helps. If not we may need to dig a bit further.
