[Bug]: border-opacity doesn't work with default border color

[Nick-Mazuk]

What version of Tailwind CSS are you using?

2.1.1

What build tool (or framework if it abstracts the build tool) are you using?

play.tailwind.com

What version of Node.js are you using?

play.tailwind.com

What browser are you using?

Safari 14.1 and Brave 1.23.75 using Chromium: 90.0.4430.93

What operating system are you using?

macOS 11.3.1

Reproduction repository

https://play.tailwindcss.com/8qs2AKmp9g

Describe your issue

Border opacity doesn't work with the default border color.

For instance, I'd expect to not see a border on this div since the border-opacity is 0. However, can still see a border.

<main class='w-screen h-screen p-8'>
    <div class='w-full h-full border border-opacity-0'></div>
</main>

However, adding a border color (e.g., border-red-500) lets the opacity work correctly.

If for some reason the playground link doesn't work, just start with a new playground and replace the HTML with the HTML above.

[bradyrenting]

However, adding a border color (e.g., border-red-500) lets the opacity work correctly.

I have the exact same issue but with custom colors as well.