Problems adding custom buttons to tinyMCE editors since Types 3.4.16 update

Symptoms

Types WYSIWYG fields use the tinyMCE editor.

It is ordinarily possible to add custom buttons to such editors (through code, or such buttons may be added via third-party plugins), but these no longer appear since the Types 3.4.16 update.

Similarly, custom format buttons which can be added to the tinyMCE editor no longer work as expected on Types WYSIWYG fields.

Workaround