Steps to reproduce the issue

Install from joomla-cms/staging
Due to bugs in the HEAD Im using commit 1f89fb0 for this test

Add an editor custom field in com_fields

Edit a content item, click fields tab, see the new editor, then attempt to use an editor plugin from the bottom row

Expected result

The features work as expected, and insert their results into the Editor Custom Field area

Actual result

The features popup into a modal, and on selecting their item, the modal stays shown and the results are inserted into the editor on the Content Tab and NOT into the new editor on the field tab.

System information (as much as possible)

https://gist.github.com/PhilETaylor/9d3c6591ed3d7d5273626f8c5d7ae193

Additional comments

Tested on FRONT and BACK end