How do you avoid disrupting the flow of work when the Product Owner asks for a release?

Question

My team has been using Scrum for years with a goal to deliver a potentially releasable build each sprint. We recently started using a Kanban board moving away from time-boxed delivery to feature-boxed delivery. So far, this seems to better fit the teams natural way of working. With feature-boxed delivery, stories are pulled from the backlog when an individual feels they are comfortably ready to begin working on it.

The teams average story lead time is three weeks. So, theoretically, at the moment the product owner asks for a release, the team should be able to deliver it three weeks later.

When the PO sets the "stop development" trigger, individuals who have time are supposed to start helping with stories already underway to begin clearing out the board. The people freed up first are typically the "front-end" folks... Those more skilled at analysis and development. They are typically asked to help with back-end work, like system testing.

The teams work flow, or cadence changes each time the stop trigger is set. What do you do to minimize this change and keep the team working as efficiently as possible? How do you avoid the disruption caused by the Product Owner asking for a release?