I would suggest slowing down and gradually raising the speed of the roll without syncopation (ideally, with a metronome). You want to be able to roll with or without syncopation at will, so definitely don’t completely eradicate syncopation every single time you practice.
Also, be sure to listen to songs with the banjo rolling “straight” as much as you can– the really hard driving, faster stuff is usually where this happens the most (as opposed to a more swingy-folky sound). Some players, even while playing fast, will subconsciously swing/syncopate the rolls. At least, I assume it’s subconsciously! You can hear this sometimes when you slow a really fast song down.
Also, check out the Shuffle Rhythm lesson: