I'm having this same issue...and have been unable to find a fix. one of the clients sees it,..I have not been able to replicate....
I've tested HTML5 Doctype, and important...gmail app doesn't seem to be supporting styles outside of the media query...it works on the Inbox App, and the Android app but not gmail...Basically I have a style class justhide, which house display: none on a preheader....it disappears on desktop, Inbox app and Android app, but Gmail it just stares at me. I'm doing this on a Samsung Galaxy 7 Edge.
I'm kind of afraid that'll roll out at midnight or something and we'll wake up to calls from angry clients.
They need to make sure they are pasting the code in the source tab of the delivery or block. Never the HTML tab. The HTML tab is a wyiswyg editor and by Adobe's documentation should never be used for editing because it tries to recode the email.