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SharePoint uses different response lengths for external result sources (such as the Confluence result source) and the internal result sources.
To increase the length of the result source, you will need to run the following power shell script on the SharePoint Server that your Search Applications runs on. You will also need to change the value of $sourceName
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If the error still persists, you need to get rid of SP cache by reseting resetting IIS of the query server.
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