The oil seals on modern jet engines are "labarinth" type seals which rely on air pressure to counteract the oil pressure. During start up , you can get the oil pressure coming up quite quickly , way before full balancing air pressure across the seals on the bearings.......so some of the oil leaks across and usually this air is vented to the turbine area so this air/oil mixture ends up at the turbine area where it gets burnt with a momentary puff of smoke.......once the core engine is up to speed the air pressure can counteract the oil pressure and so the smoke doesn't happen.
It sometimes is more obvious if the oil level has been over -serviced.