It is even a browser bug can not be named. This feature.
In short, the problem is that not having a meta tag viewport, and links do not specify font-size (at least relative, in em or rem).
The page is considered not mobile-friendly. The browser in such cases can increase the font size to make it easier to read. But the size of indents (you is 2em) the browser does not recalculate and it turns out that the font size seems normal, but the size of the indentation - reduced (in fact, the size of the indentation is normal and the font size increased for convenience).
Once you interact with the element (a link in your case), there is a redraw, the browser understands that the page looks good with the current font size sets and have a real indentation.
The screenshot below shows that the bottom two links (I interacted with them) indented margin much more than the first two.
If it is difficult to understand, then just put his body in styles font-size and everything should fall into place. :)