I'm playing a fighter with the Two-Weapon Fighting fighting style. Several times in the game tonight, I killed my target with my first attack, and so I wanted to use my bonus attack to go at one of the other monsters in the room. Once, I hadn't moved yet and so I used my movement to close with the second monster, and another time the second target was already in range without me needing to move.
Each time, however, our DM made me take a -3 to my attack roll on the second attack (he didn't even make me roll with disadvantage, just assigned an arbitrary numerical penalty) and said it was because I was switching targets.
I played a little of older versions of D&D growing up, but only really got into it a few years ago with 5e. Is this some old-school rule the DM is citing?
I've seen discussions online saying you can move between attacks, but haven't been able to find anything to suggest you take a penalty for switching to a new target.
This DM also isn't just capricious or vindictive – he always happily relents if we show him a rule in the PHB that goes against what he's saying, for example, and any homebrew/house rules get discussed beforehand. This is just the first time I've ever encountered this sort of penalty, and I've played with TWF with other DMs in the past...