We have the same issue, but mine is 1.5 years later than yours. An unreliable quirky workaround is to NOT import the gif. I have an animated gif called "rabbit.gif". Instead of importing that gif to the stage, import a jpg to the stage with the same name: "rabbit.jpg". After publishing, open the html file in an editor and find/replace "rabbit.jpg" and change the ".jpg" to ".gif". Make sure your "rabbit.gif" is in the images folder. Then delete your "rabbit.jpg" file from the images folder.
You'll find this works. You can tween it too. What you'll find quirky is when you refresh the page. I can't get it to restart the animated gif. I've tried tweaking the code a bit, but I'm just a light coder dude and haven't solved it.
I believe the hangup is Animate is trying to synchronize timing of the different elements. And doesn't think to restart the gif. I've tried having the gif start on frame 2.. and also dropping it on the last frame. That didn't work.