I do not believe Flickr embed was ever supported natively.. So I assume you are having an issue with a plugin. What plugin are you using for FLickr Embeds? Did you contact the author about the problem yet?
It’s been a standard feature of WordPress for quite some time. See here: https://en.support.wordpress.com/images/flickr-photos/
My bad. Well, then I would create a bug report at https://core.trac.wordpress.org/newticket.
Moderator
Jan Dembowski
(@jdembowski)
Forum Moderator and Brute Squad
You can also embed your Flickr images in your WordPress.com blog. To do this, select one of your Flickr images, click the “Share this photo” icon, select “Link” and copy the link
That works on WordPress.COM, for a self-hosted installation you may need a plugin.
