It's by design that SharePoint will automatically encode links in the Quick links web part. The special character "@" is replaced by "%40".
If you click the link in the Quick links web part, it will redirect to the correct TikTok video, just ignore the encode behavior.
As a workaround, you could use URL shorten service to shorten external links, then add shorten external links into Quick links web part.
Please refer this article: https://www.webbtech.org/2018/11/use-mailto-and-other-link-types-with.html?m=1
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