i'm also wondering about this, let me know if anyone figures it out

It can be removed by creating a CSS snippet with this content

.view-header {
    border-bottom: none !important;
}

I am no CSS expert, but this is the snippet I stated using to remove the dashed line:

body.anp-card-layout {
    --file-header-border: var(--border-width)
    var(--background-modifier-border);
}

But @DomizianoScarcelli snippet also seems to work. So those are two ways to remove the dashed lines. Hopefully in future releases, the ability to remove the dashed line can become a core option for the theme.