This is a generic issue caused by a clash between state machines trying to define (or failing to define) the #open method on adjustments as part of their state changes interface. This method is already defined in Object. For more details, see: bb42e33bf7/lib/state_machines/machine.rb (L323-L350)
This used to happen via an after_save callback in Shipment, which called `order.update!`. That has recently been removed. After changing a shipment's selected shipping rate (shipping method), we need to ensure the order's totals and states are updated. We don't need to update all of the order's adjustments though (see previous commit).
FYI Spree handled this issue like this: 24388485ed, but there are lots of things about that commit that are clearly awful, like: params handling in a model, duplication of Order::Updater logic across the codebase, the Shipment class having responsiblity for knowing which things need to be updated on the order, etc. The result is ultimately the same as what we're doing here though.
