For all I know the Instagram app is NOT a native iPad app, it is an iPhone app that can be used in the iPad because they use the same OS, so you can see it at full screen in the iPhone but on the iPad it is full screen with black bars on the sides, it always been like this, if my mind does not trick me. I have iOS 12.1 on my iPad and my iPhone and no problem at all with Instagram. Check out the images and compare, with the black bars is iPad, the other one is iPhone. Now check the iPad version and see in the right bottom corner the 2 arrows pointing each other, you tap on that and scale the image to full screen or reduced screen.

