In the first PIs after we started moving, this was the case for us too. But it got better every PI. The last few PIs I was in had 10 teams and a couple of them would be playing catchup, but most would be working on other unplanned projects. Either refactoring or learning or getting extra things done.
We do PI planning from SAFe. We would get the last sprint of quarter for slack, improvements, they said.
Instead, we use it to clean up the mess created from waterfall scrum in the previous sprints.