Undetermined starting date, planning backwards seems completely messed up

Hi, I want to define the following process:

  1. I want to place the final milestone at time T, e.g., a delivery due date.
  2. I have a data collection task that must start no later than T-10w and finish no later than T-8w
  3. I have a second data collection task that must start no later than T-5w and finish no later than T-3w.

Now, when I get a delivery date, say April 1st, I want the datacollections tasks placed at the right time, i.e., April 1st - 10w and April 1st-5w, respectively, with corresponding finishing dates.

What I have done is to define the process with undetermined dates, backwards, and above described.
When I then replace in the project inspector the undetermined with a concrete date in the future, say April 1st, everything is messed up and the data collection processes appear at TODAY-10w and TODAY-5w, rather than April 1st-10w, etc.
The only thing that gets placed correctly is the single milestone at time T.

Even worse, when I go to the project inspector and click from undetermined to determined, set date to April 1st, and click then back to undetermined, the original process is completely messed up.
For example the first data collection is not anymore at T-10w (T-70d) but at T-120d!!

What am I doing wrong?
I have tried this with omniplan 3.14.4 and 4.6.

Thanks in advance