Same issue here - I have the following four iOS devices in use, all running on 11.4:
iPad Air 2 128GB: Badly affected by WiFi drops and reconnect problems
iPhone 6 16GB: Occasionally affected by WiFi drops and reconnect problems
iPhone 7 128GB: Not affected
iPad Pro 256GB: Not affected
The WiFi reconnect problems occured after upgrading to iOS 11.4!
The interesting thing is, that on the first two devices, the connection drops after about 10 seconds despite having connected succesfully. This can also be seen in the log of the router. The devices usually reconnect after roughly a minute and then stay connected until you go into screen saver mode or turn off the device.
Another problem that appears on all 4 devices since upgrading to 11.x, is the fact that the devices drop their WiFi connections a few seconds after the screen lock becomes active. This means that unless you have a mobile data connection, push mails and instant messages are being received with a huge delay. I have an old iPhone 5S that still runs on iOS 10.3 and that device is not affected by any of the above mentioned WiFi issues.