Forms are not customized on on-premises

If after Microsoft's updates, forms are not customized anymore, the following workaround can be used:
Copy the following code snippet and insert it into the first empty content editor on the form. Click inside the Rich Forms Content Editor and use "Edit Source" command from the FORMAT TEXT ribbon to paste the snippet:

Code:

<img src="/" 
onerror="this.parentNode.removeChild(this);if(window.ArdCustActn)return;window.ArdCustActn=true;var 
s=document.createElement('script');s.setAttribute('type','text/javascript');s.setAttribute('src','[Site
URL]/FormsStoreLibrary/Scripts/ArdeviaHostScriptAction.js?
v=1461170646729');document.getElementsByTagName('head')[0].appendChild(s);" /> 

Note: replace the [Site URL] with the url of your site.