I try to attack with the moon dagger, and it automatically switches it to my off hand, where I can't attack with it. I'm using the mod as part of the Roguelike Adventures & Dungeons 2. May also occur with other daggers, haven't tested it.

This is a feature for Daggers, once you land an attack it immediately switches places with your offhand weapon. I believe the intention is for Daggers to facilitate rapid weapon-swapping like you would see in PvP fights.

Closing as I personally verified that the moon dagger, as well as other dual wieldable weapons, are working as intended.