Hi Mathan ramalingam ,
Welcome to Microsoft Q&A platform and thanks for posting your question here.
As I understand your issue, sink order is not populating from the dropdown in case of multiple sink in mapping dataflow. Please let me know if my understanding is incorrect.
I tried to repro your scenario and faced the same issue. I am taking this forward with the internal team and will keep you posted once I hear back from them regarding the resolution. Thankyou.