This is interesting work! Did you happen to do any testing with VML full width backgrounds?
Might want to get Litmus to confirm this, but I believe the Gmail app previews are using a POP3/IMAP mailbox and the changes have not been made to these account types yet. I'm not sure if its planned or Gmail accounts are being prioritised first.
I was referring to the original CSS snippet of the OP in my original response, the meta tag will work across lots of clients, but its not recommended as it disables functionality on data links entirely.
The Gmail CSS changes are being rolled out gradually across the platforms.
https://litmus.com/blog/gmail-to-support-responsive-email-design
I believe if you use a POP/IMAP mailbox with the Gmail App you will not get any of the CSS3 enhancements for now, its not been planned for such accounts to support it initially, that may change in the future.
Be advised, that will disable any functionality of clicking/tapping on a phone number or other pieces of data like mailing addresses, which is often useful for interaction on mobile, CSS is often the better approach to use while keeping this functionality.
I believe that snippet is limited to iOS/Apple Mail email clients.
For Gmail you might want to try the more generic span wrapping approach:
https://litmus.com/blog/update-banning-blue-links-on-ios-devices
With CSS head support the .appleLinks classes should work. Also ensure you are inlining you're <a> styling to cover other clients.
That maybe, just maybe that on the heels of Google and Gmail making a big step forward in the email development world, Microsoft and Outlook will follow suite.
I have hope that the recent established partnership between Litmus and Microsoft will lead to my Lumia 950 XL and other Windows 10 devices actually being able to have something I could actually call an "email experience" along with actually being able to see what a mobile optimised email campaign looks like on a Windows device (its a rare sight, trust me)! I think it fair to say the Outlook clients prior to 2016 and Windows 10 are stuck where they are, but Outlook Mail can do a Gmail and have the Word engine radically overhauled. Think about it, the Word engine supporting CSS3?! I'd take that as an early Christmas present or New Years resolution any day!
To complete the statement:
“2017 will be the year of Outlook Mail".
Don't let me down Microsoft!
Started a new discussion: Do you have a minimum/maximum character count for your email preheader?
Yep, this behaviour been occurring on Office 365 for sometime and now Outlook.com inherits the same quirky behaviour, because most of the Outlook.com users have been migrated over to the Office 365 infrastructure which means you get the lovely Office 365 preprocessor free of charge (lucky you!).
https://github.com/hteumeuleu/email-bugs/issues/10
https://litmus.com/community/discussions/4594-outlook-365-displays-tbd-on-empty-href-tags#comment-6585
The preprocessor is triggered when the href value of an <a> is not the typical http:// or https:// format and when there is a special character like $, #. It won't happen when a proper href value is set.