To show the tab to the user, tab requires 'show' class to actually show, otherwise it stays hidden.

That doesn't happen on the page load on the first tab, so the first tab will be visible only after you click on the other one and then back on the first, when it finally gets the 'show' class.

You can see it on the modularcode page at "Pill tabs" section.
https://modularcode.io/modular-admin-html/cards.html

I haven't looked for the solution yet. (Except of course using JS on the client side to add 'show' class to the first tab pane.)