Right, we no longer support the box style categories as a Discourse feature… so it’s not expected to work everywhere. It is possible to add custom email styles via admin/customize/email_style/…
Adding something like this there, for example, could get you closer to the box style in email
[data-category-id] {
background: white;
color: #333 !important;
padding: 0 .25em;
span > span { // hides the bullet category badge
display: none !important;
}
}