Cursor is not at the end of the line, but behind some special characters (regex) like closing parenthesis: "def f(x: stri|):", completion should give "ng):" and replace the rest or the string.

From VS Code the regex is:

let right_of_cursor_has_only_special_chars = Boolean(right_of_cursor.match(/^[:\s\t\n\r(){},."'];]*$/));

The other keyword is "multiline=False", the model switches to a special mode to replace the rest of the current string.