It could be that the way BP is interacting with the page is putting the web app into an unexpected situation. The wen app will have been designed with the expectation of human input via keyboard and mouse. In particular the HTML Write stage does not use the keyboard, so it could be that the web app is not 'aware' that data has been entered, ie client-side scripts normally set off by the user are not being triggered.
A quick test would be to populate the page manually and then have BP click the button. If it works, then it indicates that the BP data entry logic needs adjustment.
