The first time I got an overleveled weapon was when I got a level 12 dagger at level 9, from a quest that didn't require too much combat. Was rather disappointed when I noticed the penalty. In most cases getting overleveled items means beating overleveled enemies, and I found that was pretty hard on tactician. You'd expect to get appropriate rewards for pulling that off, but that's not quite the case.
Part of the reason here, I think, is because if you play an optimized lone wolf duo/solo, it wouldn't be too hard to kill overleveled enemies and get overleveled items. And overleveled weapons make combat much easier, somewhat ruining it. The thing is, optimized lone wolf duo/solo is already broken to begin with. That being said, I haven't played the game in a long time and don't know what it's like now anymore.
Otherwise, if you're playing a non-lone-wolf party on tactician, don't bother to try to get overleveled items from combat.