Okay, found a way to reproduce it:

1. Add a new reference to a statement, use a monolingual text property as second property together with a long string.
2. Remove the first property from the reference.
3. Notice that the widget for monolingual text isn't directly under the input box and stays on the same position, even if you empty the input box

I've also noticed that the input box doesn't change size when text is going to span multiple lines if you just type.