I have tested it with Portuguese (Brasil) and French languages.

When you type an accented char, like ´ + a, you shall have a á. But if you type it fast, you will end up with a a (without the accent).

This happens for any TextInputControl, such as TextField and TextArea, and for WebView control.

You can reproduce this with a simple FXML sample in netbeans, just open the FXML, put a TextArea control and run the sample. No code associated, no event handler, you don´t even need a FXML Id.
If you type the accent, wait one second then type the letter, it works. If you do it fast, like normal fast typing, it does not.

You can test with Portuguese (Brasil) with the accents: ´ ` ^ ~ ¨ and the vowels.

I have written a KeyEvent listener and all the events where there, so I just used KeyPressed to store the accent and KeyTyped to replace the non accented char with an accented char whenever the API does not do it right and it works.

Maybe the API is using the keyReleased event and it does not happen in able time so the KeyTyped works as it should.