uhm, dude? you have a sentient gem in the envenomed blade. Doesn't matter if it has 0 xp, it's still sentient and has 1 available filigree slot. Sentience will never allow you to put it into your offhand. Remove the Gem of the Hopeful. As long as that is in that weapon, it is restricted to main hand only. That is not a bug. That is WAI.