Welcome to Microsoft Q&A, and thank you very much for reaching out.
Based on your description, may I confirm that you are filtering or searching emails by category using the built‑in filter or category list in Outlook Classic?
If so, from my research and testing on my end, this behavior is expected. In Outlook Classic, the category list shown in the filter or dropdown menus is limited (typically up to around 15 categories) and is based on recently used categories. Unfortunately, Outlook does not provide a supported option to force all categories in the mailbox to appear in that list.
That said, I found an external thread that discusses this behavior and a possible workaround, which you may find helpful. Feel free to check it here.
Note: This information is provided as a convenience to you. These sites are not controlled by Microsoft, and Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please ensure that you fully understand the risks before using any suggestions from the above link.
I also recommend submitting your feedback through the Microsoft Feedback portal, where Microsoft engineering team actively collects customer input to help improve the product. The more users share the same feedback, the higher the chance it may be considered for a future update.
Thank you for your understanding.
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