[4.1] Editor custom field 'Safe HTML' setting not working correctly with iframes

Steps to reproduce the issue

• Create an article custom field of type Editor
• Select 'Safe HTML' from the Filter options at the bottom of the General tab and save the new field
• If using TinyMCE, ensure 'iframe' is removed from the list of prohibited elements in the plugin settings
  or if using JCE Editor ensure that all iframes are allowed in the profiles editor under Plugin parameters > Media support
• Open an article that uses that field and select the field
• Switch to the Code editor and paste in some html, e.g. <p><iframe src="https://www.google.com/maps/embed?pb=[google-maps-link]" width="600" height="400" style="border: 0;" loading="lazy" allowfullscreen="allowfullscreen" frameborder="0"></iframe></p>

Switch back to the WYSIWYG editor
Save the document

Expected result

The map is saved and visible

Actual result

TinyMCE:
Switching from Code to Editor shows just a blank box, then even before saving the article the code is reduced to
<p><iframe></iframe></p>

JCE Editor:
The map is visible in the Editor pane prior to saving the article, after which the code is stripped out entirely

System information (as much as possible)

J4.1.2

Additional comments

It does the same with the 'RAW html' setting

Start by reproducing the issue in the Editor custom field with the Safe HTML and RAW html settings, testing both the TinyMCE and JCE Editor paths described. Compare the iframe content before switching editors and after saving; done means the iframe remains intact and the embedded map is visible after saving.