Thanks for the video, very helpful!
The issue is that when the plugin adds the CSS for border-color, it uses the incorrect color format, while it uses the correct color code for the background.
Please reach out to the plugin’s support for this issue, as GP has no control over how the plugin generates CSS.
In the meantime, you can use this CSS before they get back to you:
.jetpack_subscription_widget input#subscribe-field-2 {
border-color: var(--global-color-8);
}