I tried your source code on my own site. Adding <meta name="viewport" content="width=device-width, initial-scale=1"> to the <head> section makes the text sizing constant, regardless of the lines.
By the way, including this in the html head is pretty much the most fundamental thing you need to do when developing a site with mobile users in mind. This line tells the browser the basic scaling of the site.
This link from mdn web docs pretty well describes why it works that way.