LTE cannot accept voice calls, so it switches to 3G (i.e. "4G" on AT&T) to take the call. It should switch back to LTE after a few minutes. You can try toggling the Airplane Mode on and off if it doesn't.
That said, I did notice it taking a little while over the weekend, though now it seems better. I wouldn't be surprised if a future update addresses handoffs between 3G and LTE. What carrier do you use?