You're right, there are specific Outlook issues with buttons, smart elements (roundings, hidden elements, highlighting, etc.), pictures and background images displaying. The matter is that service does not support indentation and media styles.
For more details about all the Outlook "pitfalls", click here.
For this specific service, we recommend you to set color not for the button block but to the entire container.
For example, we have a button that looks like this in our editor...
...but in Outlook it loses its style.
To fix the issue, go from block to container...
and choose the same color and borders as the button's.
This is our result with Outlook.