I've had some functionality of the Text Area widget not working for me (counting the amount of characters written in the box). And in the end we contacted Mendix about it.
The reply we received from Mendix mentioned that they were rewriting their widgets and that for the moment you need to 'opt-in' to get the new widgets which DID work as expected for us.
I'm not sure if this will fix your particular issue. Because it's not the same as mine. But you could try the following:
Create a boolean constant named EnableNewTextArea in your project and set it to true.
Then rerun your project which should activate the new widget.
This fixed our issues with the text area widget.