Think I have a fix for this now! Will put it in for the next release. As a temporary solution, you just need to add the following code as soon as possible after creating the Textbox (either Begin Experiment with the Code component below the Textbox in the Routine order, or Begin Routine):
myTextbox.container.disable()
It was working in our test scripts because, once the Textbox has been drawn once, the Aperture is correctly enabled/disabled so that it only applies to the Textbox. But our tests didn’t take into account the period before the Textbox has been drawn - which is what’s causing the problem here. So all it needs is disabling on creation so that there’s no Aperture during that time.