diff -u b/core/modules/picture/lib/Drupal/picture/Entity/PictureMapping.php b/core/modules/picture/lib/Drupal/picture/Entity/PictureMapping.php
--- b/core/modules/picture/lib/Drupal/picture/Entity/PictureMapping.php
+++ b/core/modules/picture/lib/Drupal/picture/Entity/PictureMapping.php
@@ -95,23 +95,16 @@
     if (isset($this->breakpointGroup) && is_object($this->breakpointGroup)) {
       $this->breakpointGroup = $this->breakpointGroup->id();
     }
+
     // Split the breakpoint ids into their different parts, as dots as
     // identifiers are not possible.
     $loaded_mappings = $this->mappings;
     $this->mappings = array();
     foreach ($loaded_mappings as $breakpoint_id => $mapping) {
-      $map = &$this->mappings;
-      foreach (explode('.', $breakpoint_id) as $id) {
-        if (!is_array($map)) {
-          $map = array();
-        }
-        elseif (!isset($map[$id])) {
-          $map[$id] = array();
-        }
-        $map = &$map[$id];
-      }
-      $map = $mapping;
+      list($source_type, $source, $name) = explode('.', $breakpoint_id);
+      $this->mappings[$source_type][$source][$name] = $mapping;
     }
+
     parent::save();
     $this->loadBreakpointGroup();
     $this->loadAllMappings();
@@ -146,21 +139,14 @@
     $this->mappings = array();
     if ($this->breakpointGroup) {
       foreach ($this->breakpointGroup->getBreakpoints() as $breakpoint_id => $breakpoint) {
-        // Get the mapping for this breakpoint id.
-        $mapping = &$loaded_mappings;
-        foreach (explode('.', $breakpoint_id) as $id) {
-          if (isset($mapping[$id])) {
-            $mapping = &$mapping[$id];
-          }
-          else {
-            $mapping = array();
-            break;
-          }
-        }
+        // Get the components of the breakpoint id to match the format of the
+        // configuration file.
+        list($source_type, $source, $name) = explode('.', $breakpoint_id);
+
         // Get the mapping for the default multiplier.
         $this->mappings[$breakpoint_id]['1x'] = '';
-        if (isset($mapping['1x'])) {
-          $this->mappings[$breakpoint_id]['1x'] = $mapping['1x'];
+        if (isset($loaded_mappings[$source_type][$source][$name]['1x'])) {
+          $this->mappings[$breakpoint_id]['1x'] = $loaded_mappings[$source_type][$source][$name]['1x'];
         }
 
         // Get the mapping for the other multipliers.
@@ -168,8 +154,8 @@
           foreach ($breakpoint->multipliers as $multiplier => $status) {
             if ($status) {
               $this->mappings[$breakpoint_id][$multiplier] = '';
-              if (isset($mapping[$multiplier])) {
-                $this->mappings[$breakpoint_id][$multiplier] = $mapping[$multiplier];
+              if (isset($loaded_mappings[$source_type][$source][$name][$multiplier])) {
+                $this->mappings[$breakpoint_id][$multiplier] = $loaded_mappings[$source_type][$source][$name][$multiplier];
               }
             }
           }