Created attachment 400782 [details] CSV file (tab separated) to reproduce the issue. When previewing a downloaded CSV file which is UTF-16LE encoded in Safari on iOS which contains Chinese characters, Safari incorrectly renders the contents of the file. This does render correctly in Excel on iOS and in all other major browsers and text editors. Attached is a file that you can use to reproduce the issue. You will need to write a small HTML application that will download the file on a click event. When Safari prompts you to view or download the file, click view. Please note that the file does contain a byte order mark (BOM). Expected The file renders Chinese characters correctly. Actual The file renders garbled text.
I am fairly certain that this issue in not in WebKit, but in an Apple system framework used for previewing downloads. Could you please report it to Apple via https://feedbackassistant.apple.com? The same issue reproduces when previewing the file in Finder on macOS.