When using the .setText(String text) method for a TextDisplay, it puts the string given into a format that does not change the text of the TextDisplay. The 'text' nbt looks like this after the setText method has been used like textDisplay.setText("This is a test");

text:'{"extra":[{"text":"This is a test"}],"text":""}'

This is not correct, as it should put the text like so:

text:'{"text":"This is a test"}'