Hi,
Usually this turns out to be tracking. When a sales line has lot or serial numbers on it, posting stops looking at Qty. to Ship and follows the Tracking Specification instead. If the tracking was entered for the full line quantity, the full quantity ships, no matter what Qty. to Ship shows.
The line that backorders correctly is probably the one without tracking, or the one where tracking already matches the partial quantity. The others most likely have tracking for the whole amount, and posting just honours it.
I'd compare the tracking quantity against Qty. to Ship first. Open the line, go to Line, Item Tracking Lines, and have a look. If tracking is bigger, this is the cause. Reducing the tracking to match what really ships solves it.
Same logic for reservations. A line fully reserved that nobody trims will post on the reserved quantity.
Worth checking the location as well. Seen this a few times on warehouse-managed locations, where Require Shipment or directed put-away and pick is on. There, the Qty. to Ship on the order is basically ignored, the shipped quantity comes from the Warehouse Shipment or the registered pick.
Default Quantity to Ship set to Remainder is a global setting, so this part is not the issue. As a quick test, skip the Release step and post directly after entering Qty. to Ship. If it works, Release is the one pulling tracking or reservations back to full quantity.
In most cases, a screenshot of the Item Tracking Lines window before posting closes the case.
