You could follow below tips to troubleshoot this issue.
1.Check the folder whether has unique permission and users whether have permissions for the folder.
2.Go to Library settings -> Advanced settings, set "Allow non-default views from this list to appear in search results?" as "Yes", then click Reindex Document Library.
3.Other contents in this document library display in the search results?
4.Try to create a new folder in a new document library, search, compare the results.
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