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Dear Thomas,
Good day!! Thanks for posting your concern in this Microsoft Community.
As per the description shared, we understand your concern and would like you to check whether you are able search via KQL, here is the reference article: Keyword Query Language (KQL) syntax reference | Microsoft Learn
If still the same problem, it seems the problem could be caused due to the indexing and I would like to suggest you create a support ticket from the Office 365 admin center where the related team can investigate further. Reference: Get support - Microsoft 365 admin | Microsoft Learn
Appreciate for your patience and understanding. Have a good day!!
Sincerely,
Mia | Microsoft Moderator