Background image does not stretch fully in Outlook and Windows Mail - css

Question

I'm having this extremely odd problem with creating bulletproof background images, and this thing is driving me nuts.
Issue:
There are two variable content blocks and they include code that makes a background image appear on Outlook 2013 and Windows 10 Mail.
While the background images do show, the v:rect created seems to refuse to take up the 800 px width it's supposed to. See attached for a demonstration of how it looks in Microsoft Outlook 2013.
It shows fine in Outlook 2010.
Code for the first block (which is pretty much suffering the same issue as the 2nd) for brevity:
<!-- // Begin Module: Welcome Block \\ -->
<table align="center" border="0" cellpadding="0" cellspacing="0" class="wrapper_table" mc:repeatable="layout" mc:variant="Welcome Block" >
<tbody>
<tr>
<td bgcolor="#3b3f40" style="background:rgb(59,63,64) url('https://gallery.mailchimp.com/0728d953751ef38036e722a85/images/0ecfdaa9-07ef-4f83-8e45-ef4ed827bcb0.jpg'); background-repeat:no-repeat; background-position: center top 100% 100%; background-size: 100% 100%; width:800px; max-width:800px;">
<!--[if gte mso 9]>
<v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="width:800px; height:340px; mso-position-horizontal:center;">
<v:fill type="frame" src="https://gallery.mailchimp.com/0728d953751ef38036e722a85/images/0ecfdaa9-07ef-4f83-8e45-ef4ed827bcb0.jpg" color="#3b3f40" />
<v:textbox style="mso-fit-shape-to-text:true" inset="0,0,0,0">
<![endif]--><div>
<table cellpadding="0" cellspacing="0" border="0">
<tbody>
<tr>
<td class="content" style="width:800px;">
<table cellspacing="0" cellpadding="0" border="0" width="100%">
<tbody>
<tr>
<td class="padding" style="width:20px;">
</td>
<td class="content_row" align="center" style="width:760px;">
<table cellpadding="0" cellspacing="0" border="0" align="center" width="100%" class="mobile_centered_table">
<tbody>
<tr>
<td class="section_h" height="90">
</td>
</tr>
<tr>
<td class="content_row" valign="top" align="center" style="color:#ffffff;font-family:Montserrat, Arial, sans-serif;font-size:36px;font-weight:700;line-height:36px;text-transform:uppercase;width:760px;">
<span class="white_text" mc:edit="welcome_header"> WELCOME TO ULTIMAIL </span>
</td>
</tr>
<tr>
<td height="15" class="space_class" style="font-size:1px;line-height:1px;"> 
</td>
</tr>
<tr>
<td class="white_text" align="center" style="color:#ffffff;font-family:Montserrat, Arial, sans-serif;font-size:16px;font-weight:400;line-height:24px;width:760px;" mc:hideable mc:edit="welcome_subheader">
Curabitur et ligula. Ut molestie a, ultricies porta urn vestibulum commodo
</td>
</tr>
<tr>
<td height="30" colspan="3">
</td>
</tr>
<tr>
<td valign="middle" align="center">
<table cellspacing="0" cellpadding="0" border="0">
<tbody>
<tr>
<td bgcolor="#0d793d" class="button blue_button" align="center" width="160" height="45" style="border-radius:100px;display:block;background-color:#0d793d;border-color:rgb(57,57,57);border-width:0px;" mc:hideable mc:edit="a9">
<table cellspacing="0" cellpadding="0" border="0">
<tbody>
<tr>
<td style="color:#ffffff;font-family:Montserrat, Arial, sans-serif;font-size:14px;font-weight:400;text-align:center;text-transform:uppercase;" align="center">
<a class="white_text" href="#" target="_blank" style="color:#ffffff;display:block;line-height:45px;text-decoration:none;width:100%;">
read more
</a>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</td>
<td class="padding" style="width:20px;">
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td height="40">
</td>
</tr>
<tr>
<td height="45" class="nomobile">
</td>
</tr>
</tbody>
</table></div>
<!--[if gte mso 9]>
</v:textbox>
</v:rect>
<![endif]-->
</td>
</tr>
</tbody>
</table>
<!-- // End Module: Welcome Block \\ -->
See it malfunction in Outlook 2013: https://i.stack.imgur.com/mwTKx.png
See it malfunction in Windows Mail 10: https://i.stack.imgur.com/bXczG.png