It’s due to a style being applied to the card. You can work around it with New-UDStyle.
New-UDStyle -Style ".ud-mu-cardbody { display: block !important; }" -Content {
New-UDCard -Content {
New-UDHtml "Hello<br/>Goodbye"
New-UDElement -Tag 'br'
New-UDHtml "Cool"
}
}