I'm also having this issue. Looks like it's related to wkhtmltopdf rather than rotativa. Rotativa actually uses this under the hood anyway.
I've tried generating the PDF both with Rotativa and wkhtmltopdf and get the issue using both.
The solution I went for was to generate the legend manually in the view, looping through the different categories and colours like so
<table class="legend-table">
@foreach (var category in Model.Data)
{
<tr>
<td style="background-color: @category.Colour"></td>
<td>@category.Category</td>
<td>@category.Value %</td>
</tr>
}
</table>