When you place a shulker box with a dispenser and there is a comparator detecting where the shulker box is placed it will not recognise the shulker box until a block update.
The same goes for having a comparator and then breaking the shulker box with a piston - the comparator does not recognise the shulker box is gone until there is a block update.
Linked issues
Comments 3
One specified when the shulker box is opened, one specifies when it is placed. This specifies both the opening and the placing, which therefore means it is not exactly the same. Is it still a problem though? If it is, I will remove it...
Dupe of MC-107882 and MC-107639