The issue is because the
<br> tag in your code is converted to
<p> tag by the WordPress autop function. The paragraph tag in X has a base margin of
0 0 1.5em. It uses
em which is a scalable unit. Its size is based on the h2's font-size of 54px which is added dynamically. Most probably you're using the Responsive Text feature to manipulate the font-size based on screensize.
As previously said, this is because of the custom HTML and CSS you inserted in the Classic Headline Element. This is not a bug in X. Further explanation regarding why this does not work would be getting into custom development which is outside the scope of our support.
Please use the Raw Content element instead. I tested it and your code works consistently.
Thank you for understanding.