Wrong linear-gradient rendering

[sviter]

Go to https://engineering.instagram.com/let-your-code-type-hint-itself-introducing-open-source-monkeytype-a855c7284881
Link to Github is rendering incorrect. Underline is made with linear-gradient.

[jdm]

<style>
  span {
    background-image: linear-gradient(to bottom,rgba(0,0,0,.68) 50%,rgba(0,0,0,0) 50%);
    background-repeat: repeat-x;
    background-size: 2px .1em;
    background-position: 0 0.9em;
  }
</style>
<span>hi there</span>

Firefox shows a similar issue if I disable the background-repeat property, so that might be a clue.

[sviter]

If so this is #16657