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Dear Branden,
Greetings! Thank you for posting in Microsoft Community.
I'm sorry you have filtering issue in SharePoint Excel spreadsheet.
For the situation you encountered, we deeply understand the inconvenience caused and apologize for it.
If this issue only occurs in a specific Excel file, you may create a new file, copy all data from the old file and paste into new file, upload the new file to SharePoint and see if the filter dropdown works or not.
Recently, we received many reports from other community members that they encounter drop-down filter issue in Excel on the web. Considering that different users from different tenants are having the similar issue, we're afraid it may be a server-side issue which is caused from back end and needs to be investigated and fixed from back end.
The best way to troubleshoot the situation further is to report it to the related team. The more reports they receive from different customers, the more likely they will be able to find something affects users in common, do investigation and fix it soon. The support team has higher permission and more resource than us to check the issue from background. This would be the most efficient way in handing this case for you.
Microsoft 365 admin has right permission to contact related team, therefore, we sincerely recommend you use Microsoft 365 admin permission and follow steps in this article Get support - Microsoft 365 admin | Microsoft Learn, raise a support ticket and contact Microsoft related team.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Tina | Microsoft Community Moderator