I was going to comment on that but mine did exactly the same when I replaced my alternator.

If the battery is not well charged at start the lower voltage of bat will hold down the system voltage. It will take a while before normal voltage returns.

They said it should stay down regardless of RPM. If the voltage swings from RPM the VR is not doing its job

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)