SharePoint keeps checking the documents out despite the setting set to No

[iamsarah]

I have a workflow that updates some documents in a document library. The workflow runs under a service account that has full control access to the site/library. The required check out was set to No since I didn't want the documents to be checked out. Also, no metadata is mandatory, and all are free text field. 

The content type is not enabled, and all files are PDF with less than 300KB in size.  there are also less than 3000 documents in the document library. 

everything had been working for months without any issue but since Thursday a few minutes after the workflow updated the documents and finished, all the updated documents get checked out by System Account. 

To troubleshoot I created a second workflow that runs on "File modified" and checks if the file is checked out. When the first workflow updates the document the second workflow reruns False (Not Checked out) but then 3-5 min later the second workflow returns True (Checked out) and this is the account that checks out the file:

{"@odata.type":"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser","Claims":"SHAREPOINT\\system","DisplayName":"System Account","Email":"","Department":"","JobTitle":""}

Anyone has any idea what could be the issue? I couldn't replicate this issue in any other document library. 

Thanks!

[#2]

1. Verify that the check-out setting is still set to "No": Double-check that the check-out setting for the document library is still set to "No". It's possible that this setting was changed accidentally.

2. Check for other workflows or event receivers: It's possible that there is another workflow or event receiver running on the document library that is checking out the documents. Check if there are any other workflows running on the document library and if they have any actions that could check out the documents.

3. Check the SharePoint logs: Check the SharePoint logs to see if there are any errors or warnings related to the check-out process or the System Account. You can use the ULS Viewer tool to view the logs.

4. Check for custom code: If there is any custom code running on the SharePoint site or server, check if there are any methods or functions that could be checking out the documents.

5. Check with the SharePoint administrator: If you don't have access to the SharePoint logs or server, check with the SharePoint administrator to see if they are aware of any changes or issues that could be causing the documents to be checked out.

@iamsarah 

[#3]

Thanks!
the check in/out setting is still No. there is no other workflows running on the site/library.
this is SP Online and as far as I know ULS viewer just works with SP on-prem, please correct me if I'm wrong.
the report log wasn't active on the site before and now that is activated it doesn't have the past reports.
the admin team doesn't have access to the server as it's SharePoint online.

thanks!