When saved, closed and opened again, those settings are gone. (Latest version of Dorico under Win 10)
Do you experience the same?
No, not with me.
I just did a quick test, created from template a brass band project, panned the instruments alternating to 100% left or right, then saved the project and reloaded and it came back with the same state.
What if you do such simple test? Doesn’t work with you?