iOS 11 & Apple Maps: Hide Search Box in Landscape Orientation
I have this same issue. I have no solution, but believe this is a software issue that is exclusive to "narrow" iPhones (i.e., anything but the Plus) running iOS 11.

I have been using both an iPhone 8 Plus and an iPhone X for the past week, and have noticed this issue only exists on the X. On the 8 Plus, I am easily able to close the search box in landscape mode. On the X, I cannot. I have tried deleting Maps and re-installing, but with no change. It's almost as if the bottom of the search box on the X runs off the bottom of the screen, and that's the problem.

I've also tried a friend's 8, and an old 6, but running iOS 11, and both devices have the same issue -- can't close the search box in landscape mode. However, trying this on an iPhone 5 running iOS 10, and it's not an issue (in landscape mode, the box remains to the bottom of the screen, not the side, and compresses to the bottom).

It would be useful to hear from (1) people running iPhone SE or 5S using iOS 11 and (2) people running a 6/6S/7 under iOS 10 and see if this is an issue, to confirm my hypothesis.