What is your identity in your organization, member or external user(guest)?
Have you tried to use email address to search it?
Can you search your colleague normally in your client?
To exclude the client issue, we firstly recommend you try to login in Teams web client to see if this issue can be reproduced from it. If there is no issue on Teams web client, please try to clean your Teams client cache file and restart it.
If you can search the user normally using email address, please check if your full name is correct.
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