In my case, I found I needed two modifiers:

• one for the tappable shape
• second for the preview shape

// view code { }
.contentShape(RoundedRectangle(cornerRadius: 20)) // tappable shape
.contentShape(.contextMenuPreview, RoundedRectangle(cornerRadius: 20)) // preview shape
.contextMenu { }

Using just the second one didn't work for me.