I am not completely sure whether this should be classified as a bug or a feature request, but:
tiny mce plugins currently do not work in acf wysiwyg fields.
For example, if I install https://wordpress.org/plugins/vendi-tinymce-anchor/ it does not work for acf wysiwyg fields.
I think a part of the reason is that the mce_external_plugins filter is not being run in this plugin’s includes/fields/class-acf-field-wysiwyg.php
However, it seems likely that other steps taken in wp-includes/class-wp-editor.php could also be necessary.
(That said, for backwards compatibility for existing wysiwyg field users, it might be desirable for mce_external_plugins to be optional?)
Bumping this one as I’m having issues with TimeMCE plugins too.
Please help with this! This is a big issue for us. As rdm mentioned above, it appears that the mce_external_plugins filter is not being run on the WYSIWYG field. Thank you for your help with this!