Tasks getting pushed forward by unconnected tasks

Firstly I am new to this, so please forgive me if my question is ridiculous.

I have a basic project, and there is a fixed milestone. Task 1 must start 3 days after the milestone. In order to complete Task 1 I need to prepare, and this must be done as late as possible before Task 1 starts.

My project looks appears as A in the figure below, as expected.

So I now add a new task, and this one will start now and take 7 days. But when I add Task 2, Task 1 and the Prep now unexpectedly get pushed forward to end at the same time as Task 2, and the condition I set that Task 1 should start 3 days after the milestone is now ignored. My project now looks like B in the figure, and this is not what I want:

Can anyone explian why Task 1 is being pushed forward?

In reality the project I am dealing with is much more complicated than the scenario above, but the issue I am struggling with is the same.

Grateful for any assistance

@the3rdParty I believe this is happening because the “Prep” task’s ALAP scheduling is overriding the “Task 1” task’s default ASAP scheduling. Because adding “Task 2” pushes the project end date out, “Prep” is rescheduled to occur as late as possible in the project. The best suggestion I can come up with is to switch “Prep” back to being an ASAP task, and add a F->S dependency from the milestone with a 1 day lag.

If this workaround doesn’t help, our Support team would be happy to look into this scheduling scenario with you! They can be reached at omniplan@omnigroup.com.

Thanks for the reply. Unfortunately the work aroud will not really fit my requirements, mainly as the preparation phase (Task 1 in your screen shot) is of ill defined duration at the moment, whilst the actual task duration (Prep in your screen shot) and its lag behind Event A is known. My plan needs to be able to change itself appropriately to both changes in the date of Event A, and and the duration of the preparation, an hence I want the Task’s ASAP scheduling to take priotry over the prep’s ALAP.

I will try the support team.

Hi. I am a new member to this forum and I would like to thank you for your reply as it helped me too.