diff --git a/lib/helpers/common.js b/lib/helpers/common.js
index b0a8f1b1fd9..3398f4cbe09 100644
--- a/lib/helpers/common.js
+++ b/lib/helpers/common.js
@@ -33,15 +33,14 @@ function flatten(update, path, options, schema) {
     result[path + key] = val;
 
     // Avoid going into mixed paths if schema is specified
-    if (schema != null && schema.paths[path + key] != null && schema.paths[path + key].instance === 'Mixed') {
-      continue;
-    }
+    const keySchema = schema && schema.path && schema.path(path + key);
+    if (keySchema && keySchema.instance === 'Mixed') continue;
 
     if (shouldFlatten(val)) {
       if (options && options.skipArrays && Array.isArray(val)) {
         continue;
       }
-      const flat = flatten(val, path + key, options);
+      const flat = flatten(val, path + key, options, schema);
       for (const k in flat) {
         result[k] = flat[k];
       }
diff --git a/lib/helpers/updateValidators.js b/lib/helpers/updateValidators.js
index 6b68ebc9b99..199ef0677f7 100644
--- a/lib/helpers/updateValidators.js
+++ b/lib/helpers/updateValidators.js
@@ -4,7 +4,6 @@
  * Module dependencies.
  */
 
-const Mixed = require('../schema/mixed');
 const ValidationError = require('../error/validation');
 const cleanPositionalOperators = require('./schema/cleanPositionalOperators');
 const flatten = require('./common').flatten;
@@ -54,7 +53,7 @@ module.exports = function(query, schema, castedDoc, options, callback) {
         continue;
       }
       modifiedPaths(castedDoc[keys[i]], '', modified);
-      const flat = flatten(castedDoc[keys[i]]);
+      const flat = flatten(castedDoc[keys[i]], null, null, schema);
       const paths = Object.keys(flat);
       const numPaths = paths.length;
       for (let j = 0; j < numPaths; ++j) {
@@ -79,7 +78,7 @@ module.exports = function(query, schema, castedDoc, options, callback) {
 
   if (!hasDollarUpdate) {
     modifiedPaths(castedDoc, '', modified);
-    updatedValues = flatten(castedDoc);
+    updatedValues = flatten(castedDoc, null, null, schema);
     updatedKeys = Object.keys(updatedValues);
   }
 
@@ -97,12 +96,6 @@ module.exports = function(query, schema, castedDoc, options, callback) {
       return;
     }
 
-    // gh-4305: `_getSchema()` will report all sub-fields of a 'Mixed' path
-    // as 'Mixed', so avoid double validating them.
-    if (schemaPath instanceof Mixed && schemaPath.path !== updates[i]) {
-      return;
-    }
-
     if (v && Array.isArray(v.$in)) {
       v.$in.forEach((v, i) => {
         validatorsToExecute.push(function(callback) {