Updating a page via REST API causes 501 error after upgrading to Confluence 7+ due to contentproperties table modification

Issue Summary

Updating a page via REST API causes 501 error after upgrading to Confluence 7+ the first time it's called.
The propertyname = 'sync-rev-source' or equivalent from the table contentproperties is replaced with the value restored in all rows.
Viewing the page or making the call again will succeed as it sets the 'propertyname' back to 'synchrony' or 'synchrony-ack' as expected.

Steps to Reproduce

1. Install Confluence 6.15.9 and make sure the Demonstration Space was created (You can create other pages for testing)
2. Run the following query:

   select * from contentproperties where propertyname = 'sync-rev-source';

1. 1. Make sure that none of the properties is set to 'restored'. They should be either 'synchrony-ack' or 'synchrony' as described on How do drafts work.
2. Upgrade Confluence to 7.1.0
3. Make sure not to view any of the pages
4. Execute the same query again. You should be able to see that every property was set to 'restored':

   +--------------+-----------------+---------------+-----------+-----------+-------------+
   | propertyid   | propertyname    | stringval     | longval   | dateval   | contentid   |
   |--------------+-----------------+---------------+-----------+-----------+-------------|
   | 131237       | sync-rev-source | restored      | <null>    | <null>    | 65590       |
   | 131328       | sync-rev-source | restored      | <null>    | <null>    | 65628       |
   | 131291       | sync-rev-source | restored      | <null>    | <null>    | 65607       |
   

1. Pick one of the rows with 'restored' value and it's contentId and execute following REST

   PUT request: {base-url}
   /rest/api/content/
   
   {contentId} with this json body:
   {"id":{contentId}
   ,"type":"page",
   "title":"new page updated","space":
   
   {"key":"key"}
   ,"body":{"storage":{"value":
   "<p>This is the updated text for the new page updated</p>","representation":"storage"}},"version":
   
   {"number":1}
   }}
   

1. Notice the following response:

   {
   "statusCode": 501,
   "message": "Unable to save changes to unreconciled page ContentId
   
   {id=65607}
   . Refreshing the page should fix this.",
   "reason": "Not Implemented"
   }

Expected Results

The page is updated accordingly.

Actual Results

A 501 error is received and the page is not updated.

Workaround

Making the call again or viewing the page will allow it to work as it sets the stringval in contentproperties back to synchrony or synchrony-ack.

• 7.0.2 - Disabling/enabling collaborative editing clears the 'sync-rev-source' values and allows the calls to work. This method does not work for 7.1.0.

Notes

Known to cause issues with some plugins like K15 Scroll Versions when trying to publish pages, as the same error is thrown when using the JAVA API methods saveContentEntity() and saveNewVersion() from the com.atlassian.confluence.pages.DefaultPageManager class.
Calling again the methods does not always work in setting the stringval in contentproperties back from "restored" as opposed to the REST API so that workaround for plugins is not reliable.