diff --git a/addons/mail/models/mail_message_subtype.py b/addons/mail/models/mail_message_subtype.py
index ee754acdbed..a634c9d61c0 100644
--- a/addons/mail/models/mail_message_subtype.py
+++ b/addons/mail/models/mail_message_subtype.py
@@ -57,6 +57,9 @@ class MailMessageSubtype(models.Model):
     @tools.ormcache('model_name')
     def _get_auto_subscription_subtypes(self, model_name):
         """ Return data related to auto subscription based on subtype matching.
+        Here model_name indicates child model (like a task) on which we want to
+        make subtype matching based on its parents (like a project).
+
         Example with tasks and project :
 
          * generic: discussion, res_model = False
@@ -65,13 +68,15 @@ class MailMessageSubtype(models.Model):
 
         Returned data
 
-          * all_ids: all subtypes that are generic or related to task and project
-          * def_ids: for task, default subtypes ids
-          * int_ids: for task, internal-only default subtypes ids
+          * child_ids: all subtypes that are generic or related to task (res_model = False or model_name)
+          * def_ids: default subtypes ids (either generic or task specific)
+          * all_int_ids: all internal-only subtypes ids (generic or task or project)
           * parent: dict(parent subtype id, child subtype id), i.e. {task_new.id: new.id}
           * relation: dict(parent_model, relation_fields), i.e. {'project.project': ['project_id']}
         """
-        all_ids, def_ids, int_ids, parent, relation = list(), list(), list(), dict(), dict()
+        child_ids, def_ids = list(), list()
+        all_int_ids = list()
+        parent, relation = dict(), dict()
         subtypes = self.sudo().search([
             '|', '|', ('res_model', '=', False),
             ('res_model', '=', model_name),
@@ -79,15 +84,16 @@ class MailMessageSubtype(models.Model):
         ])
         for subtype in subtypes:
             if not subtype.res_model or subtype.res_model == model_name:
-                all_ids += subtype.ids
+                child_ids += subtype.ids
                 if subtype.default:
                     def_ids += subtype.ids
             elif subtype.relation_field:
                 parent[subtype.id] = subtype.parent_id.id
                 relation.setdefault(subtype.res_model, set()).add(subtype.relation_field)
+            # required for backward compatibility
             if subtype.internal:
-                int_ids += subtype.ids
-        return all_ids, def_ids, int_ids, parent, relation
+                all_int_ids += subtype.ids
+        return child_ids, def_ids, all_int_ids, parent, relation
 
     @api.model
     def default_subtypes(self, model_name):
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 2ea9f9ed872..57f47d94a27 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -2784,9 +2784,10 @@ class MailThread(models.AbstractModel):
 
         new_partners, new_channels = dict(), dict()
 
-        # fetch auto subscription subtypes data
+        # return data related to auto subscription based on subtype matching (aka: 
+        # default task subtypes or subtypes from project triggering task subtypes)
         updated_relation = dict()
-        all_ids, def_ids, int_ids, parent, relation = self.env['mail.message.subtype']._get_auto_subscription_subtypes(self._name)
+        child_ids, def_ids, all_int_ids, parent, relation = self.env['mail.message.subtype']._get_auto_subscription_subtypes(self._name)
 
         # check effectively modified relation field
         for res_model, fnames in relation.items():
@@ -2795,15 +2796,21 @@ class MailThread(models.AbstractModel):
         udpated_fields = [fname for fnames in updated_relation.values() for fname in fnames if updated_values.get(fname)]
 
         if udpated_fields:
+            # fetch "parent" subscription data (aka: subtypes on project to propagate on task)
             doc_data = [(model, [updated_values[fname] for fname in fnames]) for model, fnames in updated_relation.items()]
             res = self.env['mail.followers']._get_subscription_data(doc_data, None, None, include_pshare=True, include_active=True)
             for fid, rid, pid, cid, subtype_ids, pshare, active in res:
+                # use project.task_new -> task.new link
                 sids = [parent[sid] for sid in subtype_ids if parent.get(sid)]
-                sids += [sid for sid in subtype_ids if sid not in parent and sid in def_ids or sid in int_ids]
+                # add checked subtypes matching model_name
+                sids += [sid for sid in subtype_ids if sid not in parent and sid in child_ids]
                 if pid and active:  # auto subscribe only active partners
-                    new_partners[pid] = (set(sids) & set(all_ids)) - set(int_ids) if pshare else set(sids) & set(all_ids)
-                if cid:
-                    new_channels[cid] = (set(sids) & set(all_ids)) - set(int_ids)
+                    if pshare:  # remove internal subtypes for customers
+                        new_partners[pid] = set(sids) - set(all_int_ids)
+                    else:
+                        new_partners[pid] = set(sids)
+                if cid:  # never subscribe channels to internal subtypes
+                    new_channels[cid] = set(sids) - set(all_int_ids)
 
         notify_data = dict()
         res = self._message_auto_subscribe_followers(updated_values, def_ids)