- Dorico Pro does not automatically insert beats at the start of existing music to which you add time signatures with pick-up bars. If you add a pick-up bar at the start of a flow, that flow now begins in the pick-up bar, not in the first full bar. You can insert beats at the start to push existing music to later rhythmic positions.
In both manuals, the above paragraph is written at the end of both pick-up bar entry instructions.
The last sentence is what you did with Shift+B>1e
EDIT
There are even links to the pages about adding beats.