Endless spinning wheel displayed in some cases when adding an annotation

Steps to reproduce

1. Login as Admin/ user
2. Go to a page with some text
3. Select a word and press CTRL + M to add an annotation
4. Insert something in Annotation box
5. Click 'Add Annotation'
6. Repeat by selecting another word and add an annotation as well

Expected results

The annotations are added one after another without issues.

Actual results

Most of the times after adding the first annotation, the second one is added as well, the green 'Annotation added' message is displayed, but in the place where the annotation was added, there is an annotation box with an endless spinning wheel.

When trying to refresh the page or to navigate away, a leaving confirmation warning is triggered from the browser that the changes may not be saved.

Sometimes, the issue occurs even on the first annotation on the page, see the attached .gif file, even if the tests were done in Incognito mode (to avoid potential cache related problems).

The issue could not be reproduced on my side on XWiki 14.10.20 and 15.9, but could be reproduced starting XWiki 15.10.