Pending CKEditor changes reverted when adding new content block

When I edit a document which has a content-blocks field, and a content block contains a rich-text field, I have the following problem:

First, I edit the content of an existing (saved) rich-text field. Then, I add (or remove) another content block. Now, my edit has been reverted.

I noticed that this problem no longer exists on CMS 2.26.04 (Hippo CMS 7.9.1). I'm logging this issue just for documentation.