Webpart theme colors not working for border-color

I've got a strange situation were using the following works with background-color but not border-style.

// works as expected - picks up the current themeLight color from the site.
background-color: "[theme:themeLight, default: #c86c70]";

// doesn't work as expected - ALWAYS picks up the default color - not the themeLight color.
border-color: "[theme:themeLight, default: #c86c70]";

Is any one else seeing this? Not sure if it's due to the latest version of SPFx (1.21)? If so, I'll log another bug.