Steps to reproduce the issue

• create a custom field of type Editor (editor)
• enter some HTML code into this field for an article, e.g. <p><span style="font-weight: 900;">Hello</span> <span style="font-family: Consolas;">World!</span></p> (it doesn’t matter whether you use TinyMCE or CodeMirror)
• the HTML gets filtered
• now edit the plugin Fields - Editor and change Filter to Safe HTML
• again, enter some HTML code into the editor field (see example above)
• the HTML does not get filtered
• edit the plugin Fields - Editor again and change Filter to Text
• the HTML does not get filtered anymore, not even after saving the article nor after changing the custom field content nor for new custom fields of the type Editor (editor)

Expected result

The HTML content of all editor fields should get filtered after resetting the filte to Text. I.e. the content of existing fields should not be changed immediately after changing the filter, but on the next save of the article. The content of all new editor fields should get filtered immediately when first saving content in this new field.

Actual result

HTML content for fields of the type Editor (editor) does not get filtered anymore, neither for existing nor for new fields.

System information (as much as possible)

PHP Built On | Linux dd45920 4.15.0-128-generic #131~16.04.1-Ubuntu SMP Wed Dec 9 17:33:47 UTC 2020 x86_64
Database Type | mysql
Database Version | 5.7.28-nmm1-log
Database Collation | utf8_general_ci
Database Connection Collation | utf8mb4_general_ci
PHP Version | 7.3.22-nmm1
Web Server | Apache
WebServer to PHP Interface | fpm-fcgi
Joomla! Version | Joomla! 3.9.23 Stable [ Amani ] 24-November-2020 15:00 GMT
Joomla! Platform Version | Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent | Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36

Additional comments

Tested with German Joomla! installation package and Blog demo data.