Editing an existing text block will instead copy that block and add the edited version to the bottom of the page, leaving the original intact.
Here are some steps to reproduce this bug:
1. On a blank Lessons page, click Add Content and Add Text.
2. In the CKEditor add some text and click Save.
3. Click the “Edit text box 1” button.
4. Append some more text like “This is some additional text” and click Save.
5. Note that unexpectedly there are now two text blocks on the page: the original text block on top and beneath it a text block with the edited content.
This bug is not evident in 11.2, but seems to be a regression introduced into 11.x since that time.
This bug seems like it might be related to the fix that was applied for
LSNBLDR-722, though that’s just a guess.