Hello,
you should modify the access rule "Project/Task: employees: follow required for follower-only projects".
Something like:
<record model="ir.rule" id="project.task_visibility_rule">
<field name="name">Project/Task: employees: follow required for follower-only projects</field>
<field name="model_id" ref="project.model_project_task"/>
<field name="domain_force">[
'|',
('project_id.privacy_visibility', '!=', 'followers'),
'|','|',
('project_id.message_partner_ids', 'in', [user.partner_id.id]),
'&', <!-- And operator -->
('user_id', '=', False), <!-- no user-->
(len('project_id.message_partner_ids'), '=', 0), <!-- NO FOLLOWERS-->
'|',
('message_partner_ids', 'in', [user.partner_id.id]),
('user_id', '=', user.id)
]</field>
<field name="groups" eval="[(4,ref('base.group_user'))]"/>
</record>