I understand your incompréhension towards this situation. The Set action in a data transform SHOULD work fine as source and target properties share the same time.
As far as I am concerned, I noticed that it is not always the case, and I haven’t got any explanation about this situation either
As soon as you got a workaround, you should move on
However, I would recommend to use ‘Append to’ action so that u would not have to set any target property from the target page individually (it might get tedious if you have a lot of properties)
I tried calling same from flow post wait shape as well, it is working fine for me.
But I have seen such issues while working with waits, as the system completes the wait the following action post wait doesn’t get the proper context sometimes.
Try moving the DT to pre action if you have an assignment shape after wait, if not try adding a router utility(toworklist) after wait shape which will reset the context.
Yes, my bad, Append to is suitable for Page Lists only
However, my inital diagnostic stands, I am confident this issue is not systematic to your context and you should just move on as soon you identified a workaround