rowspan disappeared

I found a bug
I have created an article having this content:

<!-- Multiple events in a single day (note the rowspan) -->
<tr>
  <td class="agenda-date" class="active" rowspan="2">

when i Generate the static content, the rowspan property disappeares

You can see the original source here

https://wpgite.pamperoviaggi.com/gardaland-bus/

and the static content here

https://gite.pamperoviaggi.com/gardaland-bus/

Can you help me

The page I need help with: [log in to see the link]