Hi,
I ran into the following issue in Google Chrome (v61):
When

• there is overflowing content on both ("front" and "back") sides
• CSS "overflow" is not set to "hidden"

The animation does not work correctly: It shows the whole mirrored back side first and then animates the flip.
On Firefox_v55 and Safari_v9 it works as expected.

Please check the following fiddle:
https://jsfiddle.net/sjf_dev/wo3j9k54/