Hi Pranjal,
This is a custom program and it will be difficult for us to figure out the issue.
You need to check the custom logic and stuff, there is a gap in the custom logic on determining the PR doc type in the work order.
Try to debug on executing the similar and identify whether there is a gap, there can be difference on saving the work order or some field is impacting the document type.
I guess there is one scenario affecting...
Create a Work Order Save it with priority 1. Now Open the work order, change to priority 2 and add the component, (here system will choose the doc type now), dont save... now again change the priority and save, see how it triggers.
There will be some other scenarios affecting your logic, kindly check it.
Regards
Terence