There is a bug with the styling of the buttons in the Mini-Cart element. When the styling is applied, the live preview isn’t updated. However, it is being output on the front-end but the live preview is not correctly displaying the styling settings. Draw your attention to the background colour setting - see it’s set to red but is blue in the live preview.
This bug was mentioned in this post and this post but looks like it still exists.
Pro: 6.3.8
WP: 6.3