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Hi RolandsStrakis,
Thanks for sharing the additional information with us.
Report for the site activity will be displayed based on the actions done on the SharePoint site and moreover, as you mentioned this SharePoint site are never accessed after the migration that could be the possible cause for not displaying the reports. Moreover, you can give test by accessing the problematic site, uploading some files and check the behavior (per the test from Sukie screenshot and mine, the Site Activity will be updates for 48-72 hours).
At my end for one the site that which was not accessed:
At last, I will also share my test results once the Site Activity is updated.
Thanks for understanding and stay safe 😊
Best Regards,
Chitrahaas