I can't see your site (you didn't provide a link 🙂) so I can't see the colours you were using on your own buttons, but here is a CSS workaround you can try whilst they fix this.
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/* Cart Page Bugfix
  Checkout Button Styles */
button.checkout-button {
  color: #fff;
  background-color: #7877e6;
  border-color: #7877e6;
}
You'll need to substitute the colour codes that I've used for your own choices. Add it to Design > Custom CSSÂ and after saving the changes refresh your browser (F5 or Cmd-R) to see the changes take effect.
Let me know how you get on.Â
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