Spurious delay when planning with fixed milestones

We have a project, where several activities get a planned start much too late. This happened when we were trying to resolve a problem with a delayed delivery.

In short:

1. a first fixed milestone
2. a succeeding activity taking 20 days with planning as soon as possible
3. a second fixed milestone, 10 days after the first one. This one has the previous activity as a predecessor - hence this milestone will be 10 days overdue
4. a second activity, with the second milestone as a predecessor, planning as soon as possible

The Gantt-diagram shows the fixed milestone at its planned date, while planning cares about the planned data for deciding the planned start of the following activities. The attached picture shows what happens. I have been reading the code, but haven't yet found an appropriate solution. The fixed milestone should show a planned date much later (it doesn't), the diamond shown in red. The following activity should start the day after the fixed overdue milestone. Hence, it seems to be a bit more complicated to fix that bug.

A simple fix is to change all milestones to floating milestone, but add a "validated due date". Then the planning will move the milestone to planned date in the Gantt, the diamond is colored red and the dependencies become obvious.