diff --git a/addons/web/models/models.py b/addons/web/models/models.py
index 4d3c2e11daa..997d06dbcfd 100644
--- a/addons/web/models/models.py
+++ b/addons/web/models/models.py
@@ -983,10 +983,8 @@ class Base(models.AbstractModel):
         # values of the move, among which the one2many that contains the line
         # itself, with old values!
         #
-        changed_values = {fname: values[fname] for fname in field_names}
-        # set changed values to null in initial_values; not setting them
-        # triggers default_get() on the new record when creating snapshot0
-        initial_values = dict(values, **dict.fromkeys(field_names, False))
+        initial_values = dict(values)
+        changed_values = {fname: initial_values.pop(fname) for fname in field_names}
 
         # do not force delegate fields to False
         for parent_name in self._inherits.values():
@@ -999,7 +997,13 @@ class Base(models.AbstractModel):
             # fill in the cache of record with the values of self
             cache_values = {fname: self[fname] for fname in fields_spec}
             record._update_cache(cache_values, validate=False)
-        record._update_cache(initial_values)
+            # apply initial values on top of the values of self
+            record._update_cache(initial_values)
+        else:
+            # set changed values to null in initial_values; not setting them
+            # triggers default_get() on the new record when creating snapshot0
+            initial_values.update(dict.fromkeys(field_names, False))
+            record._update_cache(initial_values, validate=False)
 
         # make parent records match with the form values; this ensures that
         # computed fields on parent records have all their dependencies at
@@ -1016,7 +1020,7 @@ class Base(models.AbstractModel):
         # store changed values in cache; also trigger recomputations based on
         # subfields (e.g., line.a has been modified, line.b is computed stored
         # and depends on line.a, but line.b is not in the form view)
-        record._update_cache(changed_values, validate=False)
+        record._update_cache(changed_values)
 
         # update snapshot0 with changed values
         for field_name in field_names:
diff --git a/odoo/addons/test_new_api/tests/test_onchange2.py b/odoo/addons/test_new_api/tests/test_onchange2.py
index b3db8ef59a7..b948d8c2011 100644
--- a/odoo/addons/test_new_api/tests/test_onchange2.py
+++ b/odoo/addons/test_new_api/tests/test_onchange2.py
@@ -353,6 +353,29 @@ class TestOnchange2(SavepointCaseWithUserDemo):
         result = multi.onchange2(values, ['partner'], fields_spec)
         self.assertEqual(result['value'], expected_value)
 
+    def test_onchange_one2many_default(self):
+        """ test onchange on x2many field with default value in context """
+        default_messages = [Command.create({'body': 'A'})]
+        model = self.Discussion.with_context(default_messages=default_messages)
+
+        # the command CREATE below must be applied on the empty value, instead
+        # of the default value above
+        values = {
+            'name': 'Stuff',
+            'messages': [(Command.CREATE, 'virtual1', {'name': '[] ', 'body': 'B'})],
+        }
+        fields_spec = {
+            'name': {},
+            'messages': {'fields': {
+                'name': {},
+                'body': {},
+            }},
+        }
+        result = model.onchange2(values, ['name'], fields_spec)
+        self.assertEqual(result['value'], {
+            'messages': [Command.update('virtual1', {'name': '[Stuff] OdooBot'})],
+        })
+
     def test_fields_specific(self):
         """ test the effect of field-specific onchange method """
         discussion = self.env.ref('test_new_api.discussion_0')