What does the

target_shipment

expected to do on the line item? https://github.com/solidusio/solidus/blob/v3.3.1/core/app/models/spree/order_contents.rb#L98

This is only a thing when editing orders that are already completed. In that case Solidus needs to determine which shipment to add the quantity to. This is almost always messy; we just stopped customer service from changing completed orders and ignored the target shipment code.