I doubt there's that much you can do, other than experimenting with the mods you have installed (it's always possible that a mod has strange side effects), or loading a save game before crafting the sword and try again.
It's also possible that the game is struggling at level 100 in NG+. You could also see if there's a mod somewhere that can adjust the damage values (I've always avoided mods so I don't know if there's one).
CDPR is focussing on Cyberpunk and Gwent at the moment, so a patch seems unlikely at this point.
I'd agree that this is the most likely issue. The game was never intended for any sort of NG+. It was introduced because of overwhelming demand. The Level 100 cap was later introduced because the engine started creating all sorts of interesting weirdness at levels higher than that. (It can still behave oddly even with the Lvl 100 cap.)
So, if you have reached Level 100...you are now at the uttermost end of all things Witcher 3 for that playthrough. You are approaching the Far Lands, and all of the wonders that the Curse of Large Numbers can create.