When using a custom counter the Pages tally is 0.

Sample code below, Pages is 0 (i.e. Page 1 of 0) but if the line that increments para is removed, Pages tally is correct.

@page {
    @bottom-right {
        content: "Page " counter(page) " of " counter(pages);
        font-weight: 700;
    }
}
p {
    counter-increment: para;
}
p::before {
    content: "Paragraph " counter(para);
    font-weight: 700;
}