I have an issue with the search box as well. It is available when I first launch the client, but simply disappears after some time. Restarting Airmail brings it back. I'm on a late 2013 Retina MacBook Pro running High Sierra. The laptop is connected to a Thunderbolt dock 24/7. I think it could be happening after it goes to sleep and I wake it up, but that I'm not 100% certain. I will monitor this more closely and see if I can replicate it.