if you grab the scythe from the middle or bottom box you will not be able to equip it to the toolbelt,
only when grabbed by the top box will you be able to equip it.
Same with any tool/weapon you try to equip to it
Examples:



Edit: Never realized it, but this actually applies to equipping to specific equipment slots as well.
I usually just drag and drop equipment directly on the character to equip, so I've never actually ran across this issue.