The _read_group_stage_ids method of crm_lead decides which stages to make visible in the Kanban.

You need to override this method in your own module, removing the lines that specify to search for only the stages that match the current list of leads:

def _read_group_stage_ids(...)

    ....

    if team_id:
        search_domain += ['|', ('team_ids', '=', team_id)] 
        search_domain += [('id', 'in', ids)]   # <--- remove this line
    else: 
        search_domain += [('id', 'in', ids)]   # <--- replace this line with pass