the left hand melee weapons seem to be taking the player's block reach, instead of entity reach. i tested with pehkui mod by increasing specifically the block reach, the left hand melee was increased. i tested without pehkui to make sure it wasn't causing a problem, but i still have more reach with the left hand weapon.

Below, both weapons are in reach, as indicated:

Stepping back 2-3 blocks, the left hand still can reach: