Hi @Jiri Stransky
Per my research, there is no such function for us to retrieve results of Migration Manager. As a workaround, we can use Migration Manager reports to manage, audit and troubleshoot your migration process.
https://learn.microsoft.com/en-us/sharepointmigration/mm-reports#summary-report
You can download task level reports by following powershell script
https://spmt.sharepointonline.com/download/ReportAggregator.zip
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