CiviCRM 5.17.5 and WordPress 5.2.3 just won't work
Just seeing activity here, I realized I moved this query to chat.civicrm.org, Wordpress channel and haystack provided a general fix to civicrm core which has been in place since late fall 2019. I forgot to close things out back here on StackExchange. The ultimate problem was not that one plugin, iframe, but our theme, Benevolent Pro, was evidently calling register_hooks_front_end() before it actually should be. It was decided that could happen with other themes as well, so haystack suggested a fix to handle that case. I haven't had any problems since then. We are running Civi 5.21.3 (ESR) and Wordpress 5.4.2 currently. You can see the whole thread at:
https://chat.civicrm.org/civicrm/pl/9we4awu1c38b98eihypwdrbumh