Bug with WordPress CiviCRM 5.10.3 Remote Profiles HTML Form Snippet Form Action URL
I have the similar problem with the "thank you" page for donations. It started to displayed the same message instead of "thank you" page. We noticed, that URL is not correct: problem is caused by "&amp;amp" string in URL. When it is replaced with just &amp;, it works fine and "thank you" page is displayed correctly.