In latest iOS 15 beta preview I noticed that Tiktok.com web app is broken, there is a huge gap at the bottom of the screen all the time.
iOS 15 beta 6 Safari breaks Tiktok.com
Thanks for reporting, but the Developer forum is not the right place to report about a third party app, it is not a developer issue. You'd better file a bug report on Safari or maybe contact directly the app supplier.