Further investigation showed that the issue also occurs with Japanese input on a QWERTY keyboard. Based on this, it appears the behavior may be related to multibyte text input rather than the use of a numeric-style(10 Key) keyboard.

This issue has been confirmed to be reproducible on the following iOS versions: iOS 17.6.1 iOS 18.7.2 iOS 26.1 iOS 26.2 iOS 26.3 Beta

I tested this on the latest iOS 26.3 Developer Beta, and the same issue still occurs when using WKWebView. It also seems to affect non-Safari browsers like Chrome, which presumably rely on WKWebView internally.