I faced something similar last week. I was able to resolve it by updating the version of the chrome driver used by Appium from 2.21 to 2.25.
I downloaded the latest version of chrome driver for MAC navigated to:
node_modules > appium > node_modules > appium-android-driver > node_modules > appium-chromedriver > chromedriver > mac (copied the existing version as a back up) and then replaced the existing chromedriver file with the new version.
My webview tests started passing!
Hope that helps.