Set up as below on D365:

Set up Project X with subproject: A, B and C

Create Purchase Agreement with Default project X -> Add items line: item 0, 1, 2 and 3

Create Purchase Order with selection of that Purchase Agreement:

Add the item 0, 1, 2 and 3.

As per the actual behavior from the environment, the linkage only happened automatically for the first item/item line which is between item 0 and Project X. The second, third, .. line is not linked to the default subproject once user needs to manually reselect the correct subproject from line details. We would like to seek for development that this business requirement could be met when the Purchase Order item lines can automatically link to subprojects respectively as following scenario:

Item 0 links Project X

Item 1 links subproject A

Item 2 links subproject B

Item 3 links subproject C

All the linkage should be automatic when user creates Purchase Order.