I've got an email with rounded buttons using v:roundrect MSO specific tags. In testing all works as expected, and all versions of Outlook render this properly, including Litmus's version of Outlook 365. On my client's real life email, the buttons (along with a few other rendering issues seemingly specific to them) are very messed up. Below are some tests from Litmus's rendering of these buttons versus the real life rendering in Outlook 365.

Expected Rendering Actual Rendering

Code for button:

<v:roundrect xmlns:v="urn:schemas-microsoft-com:vml" xmlns:w="urn:schemas-microsoft-com:office:word" href="http://www.EXAMPLE.com/" style="height:40px;v-text-anchor:middle;width:300px;padding:0" arcsize="10%" stroke="f" fillcolor="#d62828">
<w:anchorlock/>
<div style="color:#ffffff;display:block;height:40px;line-height:40px">
MSO Button Text Center Removed
</div>
</v:roundrect>

I've tried adding that inner div, using a p tag, initially it had no wrapping, adjusting any number of paddings, margins, etc etc, nothing seems to budge it. The width and height in my clients real renderings is very inaccurate. My actual design has smaller buttons that again in all testing render correctly across all clients, but their computers specifically render them as:

Client Rendering