diff -upr libpyside/feature_select.cpp libpyside/feature_select.cpp
--- a/libpyside/feature_select.cpp	2020-11-11 14:51:30.000000000 +0200
+++ b/libpyside/feature_select.cpp	2021-12-01 15:39:40.356426568 +0200
@@ -405,6 +405,8 @@ static inline PyObject *SelectFeatureSet
             if (!SelectFeatureSetSubtype(sub_type, select_id))
                 break;
         }
+        // PYSIDE-1436: Clear all caches for the type and subtypes.
+        PyType_Modified(type);
     }
     return type->tp_dict;
 }