Steps to reproduce the issue

Install/enable a Captcha plugin.
Select it in Global Configuration.
Try to create a new article from frontend.
Inspect captcha field in article form.

Expected result

Captcha field shown.

Actual result

Joomla\CMS\Object\CMSObject text shown in place of Captcha field. The object appears to be an editor button:

object(Joomla\CMS\Object\CMSObject)[2425]
  protected '_errors' => 
    array (size=0)
      empty
  public 'modal' => boolean true
  public 'link' => string 'index.php?option=com_modules&view=modules&layout=modal&tmpl=component&editor=jform[captcha]&105b73bec03a5feff8c60b7b445a9e84=1' (length=146)
  public 'text' => string 'Module' (length=6)
  public 'name' => string 'file-add' (length=8)
  public 'iconSVG' => string '<svg viewBox="0 0 32 32" width="24" height="24"><path d="M28 24v-4h-4v4h-4v4h4v4h4v-4h4v-4zM2 2h18v6h6v10h2v-10l-8-8h-20v32h18v-2h-16z"></path></svg>' (length=149)
  public 'options' => 
    array (size=4)
      'height' => string '300px' (length=5)
      'width' => string '800px' (length=5)
      'bodyHeight' => string '70' (length=2)
      'modalWidth' => string '80' (length=2)