There was a bug here that had not exposed until MDL-60820 introduced the
call $mform->setDefault() at the bottom of the definition_after_data()
method. That call triggers the 'updateValue' event on all form elements.
For input text elements, this effectively sets their value to the
default one, which was empty in this case due to the way how the value
had been set.
The solution here is to set the value via the same way like the normal
set_data() method does it.
The attached Behat test reproduces the problem and makes sure the patch
works.
David Mudrák
395bca32a1