Sales order line does not split in revised order

Scenario: We have a sales order with 2 lines, line 1: order quantity 100 and line 2 order quantity :10 with shipment set as 'Default' in both the lines and Allow partial shipment as 'No' in both lines.

The inventory has 85 quantity for item in line 1. and 20 quantity for item in line 2. When we estimate the availability we see that both lines have the same scheduled ship date and the line 1 does not split

Now we create a revision of the order and remove the shipment set from both the lines (line 1 and line 2) and update the Allow partial shipment of line 1 to 'Yes".

When we click on estimate availability the line 1 is suppose to split with line 1.1 with 85 quantity which can be shipped immediately as there is on-hand available and line 1.2 to be shipped on a different date.