The multi-select field of Collaborators shows the union of Collaborators among all selected tasks but doesn’t mean each Collaborator pill shown is a Collaborator on all selected tasks. To ensure that person is, type/autocomplete their name at the end of the list and tab or return. It will look like nothing has really happened (bad UI; no real confirmation). But if you then click out of the interaction anywhere else and check that person will now be a Collaborator on all selected tasks.
Larry