If I understand everything correctly, you are talking about notifications, not pop-ups. Notifications still remain largely experimental features, and some browsers do not support them. I also assume that browsers that support them have different APIs to call them, none of which are part of the Webdriver API used by Behat.
I'm sure you can find a way for the notification to display the correct data, but you definitely won't be able to interact with it. Here is another answer that supports my thoughts.
I see that Chrome in the video complains about security, this may be a problem, why you get undefined values, the way to solve this problem is to pass this configuration to the selenium session (this is an example of Behat 3 configuration):
selenium2: browser: 'chrome' capabilities: browser: 'chrome' extra_capabilities: chromeOptions: args: - '--test-type'
A good question, however, cannot be discarded.
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