In v10, if you had a standard toolbar button, then customized it to turn it into a three-button button the previous button definition became the first (LMB) of the three-buttons.
In v11, turning a standard button into a three-button button causes the previous standard button definition to be lost... i.e. it's not added as the initial three-button.
Further, if you then deselect the three-button state for the top level button, the check mark next to the 'Three Buttons' item on the RMB menu goes away, but you're then left with an empty / buttonless three-button.