Each value for the DisabledSlots tag is supposed to disable one function of one slot, but the value 1 disables the removal of all slots, the replacement of all slots, and the placement on the main hand slot.
I believe this is not supposed to happen, as now it is impossible to make an ArmorStand that have both of these properties:
1. the player can remove and replace items on its main hand slot
2. the player cannot place items on its main hand slot
Whilst for all the other slots it is possible.
Linked issues
Comments 5
My case is not similar to Skylinerw's.
They are saying that replacing should be removed at all, but I am saying that the value 1 does more than it should (compared to other values in the current implementation).
Well, the implementation changed some time ago, so that might be why you expect different results, see @unknown's comment.
I think you might be misunderstanding (or I might be misunderstanding), but I'll edit the description to be more clear.
Something similar (if not the same) is already reported, see MC-94252.