After a successful save the page reloads with ?success=1 and a green Done! banner, and the weekday toggles, days and times reflect what was stored. If instead you see a red validation message, nothing was saved. Note the Cut-off Timer form is also embedded (as "Order cutoff") in the onboarding wizard, and saving it there writes the exact same account-wide settings, so someone completing onboarding could be the source of values you didn't expect.