In this video I go over the Xfields issue created by the latest update of Elementor ( 2.9.11)
basically if you have an Xfield inside a Button Container then the [ and the ] get converted to their respective ascii codes of %5B and %5D which then means that MPP can not read them as Xfields so they do not get converted.
The fix is to use a text field and add some standard HTML button code, see code below and the CSS to format your button as well.
Enjoy!