Game Mode: Single Player
Map: The Island
Mods: am using 20 mods in total. S+, a stacking mod (ColStax), several hotkey/QoL mods. If you end up wanting, I'll post the list.

While the Chainsaw is in my inventory (doesn't matter if item is placed in hotbar or not) and is equipped in my hands, resource weight is reduced as expected. But if I transfer the Chainsaw to a storage box without first unequipping it, the weight reduction stays until either quitting to main-menu and reloading or grabbing the Chainsaw, equipping, then unequip/switch to a different weapon/tool, etc.

While the weight reduction is "stuck", I can mount dinos and even equip other tools/weapons and keep the weight reduction. I tested with the mining drill but it does not experience the same behavior.

Obviously the workaround solution is simple (just unequip b4 removing from inventory), but there is that temptation to abuse it lol. Effectively giving 75% weight reduction for wood, etc., to every dino is quite powerful.

If this is something you would give any attention to and aren't able to reproduce, let me know if there is anything else I can provide/do. Thanks