Keep Apps on SD card
Let me tell you a bit about the Android system. When an app is updated, the developer may not merely add files to its directories, but may also delete/modify existing ones. Android system is such that, Google Play Services framework cannot update apps while they are on SD card. So, the apps are always updated by first moving them to device memory, and then modifying/adding/deleting apk files.

Unfortunately, there is no system to automatically move the apps to sd card after update. There may be apps claiming to do that, but I haven't tried any, so I cannot tell.

On all Android devices, be it 4.0-Kitkat, 5.0-Lollipop, 6.0-Marshmallow, 7.0-Nougat or 8.0-Oreo, this system is unavailable. After update, you have to manually move the app back to sd card. Nothing can be done about that.

Please note that all methods mentioned here are on unrooted phone. I don't know what can be done on a rooted one.