Text disappears when editing a document after Confluence update
We have had a Confluence Data Center installation (Along with Jira) for more than a year now, almost two years. No major issue, just the usual known confluence issues.
We recently updated our installation from version 7.17.1 to version 8.6.1 (due to several critical vulnerabilities that were reported).
Now the issue is that after the update, in most documents, when we try to edit them, certain parts of the texts (such as heading or texts before a code block) just disappears.
This is not a collaborative editing issue, as I have tried disabling the feature, only one editor actively editing and also discarding drafts.
The behavior is very, very, peculiar. Once the document is loaded in the editor, you immediately see some texts missing. If you press (Ctrl+Z) to undo any changes, all the texts that had disappeared, reappears at once. But after a few seconds or when you move your mouse cursor, they disappear again and you have to press (Ctrl+Z) to undo again.
It is a persistent issue that exists even after multiple times of saving or restoring an older version, but I cannot find any patterns as to why, or what elements of the document are affected by this. I feel like there is a script running in the background that is actively trying to format the text in the editor and that script is misbehaving. Is there any way to check this?
I have obviously tried this with different user accounts and across multiple system and the problem exists regardless.
Screen 1: Document view
Screen 2: Editor view immediately after clicking "Edit
