Email Builder - Rounded Corners Not Showing In Final Email Send

When I'm in the email builder in the design section. I wanted to give a block a background color, a border color, and also round the edges. In the builder I can see the corners are rounded like I intend to do. But in the preview and when the email is actually sent, the round corners for the border in particular doesn't stay curved it goes back to straight pointy edges. Is there a way that this can be fixed? Images attached.

Thank you!