The top and left borders of these fields are set the same way in the style, and only the top one is missing in your screenshot. That usually comes from how Chrome draws 1px lines at certain zoom or display-scaling levels. We need a little more detail to look into this.
What zoom level is Chrome set to on this page, and does the top border reappear at 100% zoom?
What display scaling does your operating system use (for example 125% or 150% on Windows)?
Do the top borders of text inputs on other pages (for example the full Post thread page or account settings) also go missing, and does it happen in other browsers?
This is an automated assessment. A member of staff may review and update it.