The design of this particular theme is that there is a theme generated “Read More” link when there is no manually added excerpt, or when the auto-generated excerpt is cropped from post content that is longer than the defined 110 words excerpt length.
When a post has a manually added excerpt, or when the auto-generated excerpt is taken from post content that is already shorter than the defined excerpt length (no need to crop the content shorter in this instance), then the “Read More” link is not present.
There is however a default WordPress “Continue Reading” link that is hidden from view by the theme, but is left in place in case anyone wants to make it visible.
The custom CSS to make this visible is like so:
a.more-tag {
display: block;
}