In fact, Dorico remembers the rhythmic position of system breaks, so when you move barlines to a different position (e.g. when you create a pickup bar retrospectively), any existing system breaks get shifted as well,
It happens even if that option in Preferences is enabled (since it doesn’t concern system breaks that are already existing)
So you can either delete the system breaks as @Christian_R is describing, or move them in Write mode: select them, set your rhythmic grid value to eighth note, and press Alt+Right Arrow: