Can not remove item from armor stand offhand, if pointing at a locked and filled slot

You can always place or remove a sword in the main hand of an armor stand, even if pointing at a filled and disabled slot by the tag DisabledSlots.

But if you try the same with a shield and the offhand of the armor stand, you can place the shield but you can't remove it if pointing at these slots.

How to reproduce

Summon this armor stand. Right-click on the body to take the sword, then right-click again to give it back. ✔

/summon armor_stand ~ ~1 ~ {ShowArms:1b,DisabledSlots:4079166,HandItems:[{id:"minecraft:diamond_sword",Count:1b},{}],ArmorItems:[{id:"minecraft:diamond_boots",Count:1b},{id:"minecraft:diamond_leggings",Count:1b},{id:"minecraft:diamond_chestplate",Count:1b},{id:"minecraft:diamond_helmet",Count:1b}]}

Summon this armor stand. Try to right-click on the body to take or give the shield, but it doesn't work ❌ . Unlike the main hand, you need to click on an area that doesn't have a disabled piece of armor.

/summon armor_stand ~ ~1 ~ {ShowArms:1b,DisabledSlots:2039583,HandItems:[{},{id:"minecraft:shield",Count:1b}],ArmorItems:[{id:"minecraft:leather_boots",Count:1b},{id:"minecraft:leather_leggings",Count:1b},{id:"minecraft:leather_chestplate",Count:1b},{id:"minecraft:leather_helmet",Count:1b}]}

https://www.youtube.com/watch?v=CxUgNc-SbQo