Manual Document Sequencing not working while Importing Orders

Dear Team,

We are trying to create manual orders as well as import orders as part of a conversion activity using an Order type which has manual sequencing enabled. But order entry form doesn't allow to enter order number, the order number field is greyed out.

If we import the order, then automatically order number gets generated while importing.

The sequence numbering profile option is set to Partial in our case. We have also tried making it None and tried creating orders, but of no use.

Can you please suggest soem pointers here? How to achieve manual sequencing while importing Sales Order?