Hello @Flo
Welcome to Atlassian Community!
Please, provide us the following information so we can properly troubleshoot why these specific issues were not migrated to the Classic project:
1 - Is there anything specific about this issue that you can see in a first look? Do they have a different issue type, links or fields different from the others that were successfully migrated?
2 - Are those issues Epics or Sub-tasks?
3 - If you repeat the migration process, did they remain in the old project?
4 - Can you please provide us a screenshot of one of the issues that were not migrated?
Let us know if you have any other questions.
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