Issue description reverts to an older version when saving without prompting the conflict

On Gitlab CE 12.3.5 (omnibus), we have been having an issue with editing descriptions since approximately 2 months (gitlab 12.1 probably).

Symptom

Basically, at some point, a user edits a description of an issue, and when he saves it, the result is an OLDER version of the description (that was at one point one of HIS versions) than the one he just saved.

It is very hard to reproduced, but we experienced it multiple times without being able to put our fingers on the exact sequence. We think that the users had the issues always opened in their respective browsers. This didn't happen within the same time frames, we are talking different independent edits that are not done simultaneously (so there is not red message of an edit conflict).

Reproducing the issue

As user A

• Press the "Pencil" to edit a description or title
• Add something (text, new paragraph, more than just whitespace or new lines)
• Don't press Cancel nor Save Changes
• Close the page by closing the browser tab (which should pop up ) and press Leave

As user B

• Edit the description and add something
• Save Changes.

As user A

• Open the issue again. See the new text from user B.
• Edit the description. See that it's your "old" description that appears.

So basically, it looks like an older version of the user B description is pulled from somewhere - it's pulled from local_cache .

And since descriptions are not versioned, there is no way of restoring back what was lost. Because of this particular issue, we lost precious production documentation that just disappeared after being edited (because you only realize you lost it after you save, and see that stuff disappeared completely).