In official scrum, the development team could choose to accept substitution. It looks like the GP's case, they are obligated to accept substitution.

After I presented the volatility findings, that changed. Sprints largely had to stay as they were and the team decided what we’d take on after the sprint started. It got better. But the whole sprint/scrum/agile thing is still weird. It can be helpful but in a large org it’s never easy. I hope for further improvement in the area of scheduling/estimation in software development.