Hi @Addison Jiang,
Unfortunately I couldn't reproduce this issue on my SharePoint Server 2019 farm. People Search suggestions didn't prompt after removing suggest phrases and the corresponding items in the database are also deleted after a while. I think maybe there is a timer job that deletes them in database but I'm not sure which one is and the running duration set in your environment. To solve this issue ASAP, open a ticket with Microsoft is recommended as MS experts can have a remote session with you and help you check the logs in this case.
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