Hi There,
Unfortunately, sticky header is a known issue for IE and it is under investigation. We’re sorry for the inconvenience.
For the missing navigation, it is because of FLEX SETTINGS for container. E 11 has some glitches in the Flex which you will need to consider.
The most important one is that flex values with no values such as auto have rendering issues in IE 11. That is why I suggest that you use percentage for flex basis options or the other related flex options. Then try to match things up with that point in mind.
Here is a non-exhaustive list of the possible rendering issues of IE11 when using flexbox:
Thank you for your understanding.