Thanks for asking.
You have updated Cornerstone but using old version of X Theme 6.0.4 and that might be the reason why mobile menu is not working. Please update X Theme to the latest version 6.2.5.
To update X Theme please take a look at following resources:
Version compatibility guide: To learn about the latest and compatible versions of bundled and supported plugins, please take a look at following doc:
2. Theme Update guide: Please take a look at following article to update Cornerstone.
3. Please take a look at following doc about changelog: