So weird, I just posted a new press release about an hour ago and I’m having the same issue - so the problem isn’t solved unfortunately 
Here’s the new page: https://sequencebio.com/press/release/2016-08-22
Again, if I use the .herokuapp.com domain, it’s fine.
Seems like there’s a big delay between the page going live and twitter finding the meta tags - maybe some sort of caching setting I’m unaware of? (the link I had in my original post was put online yesterday, and only started working a couple hours ago).