Keep Apps on SD card

I have the same behavior on my Samsung S7e - currently Android 8.0.0. However, on my Tablet SM-T900 - currently Android 5.1.1, there is an interesting features in the "application manager", which helps: amongst the 5 sheets available there, one called "SD Card" allows listing all applications "authorized" to be moved thereto, irrespective of whether they actually have been or not. So, after an apps update session, the box associated with an app which was moved away is no longer ticked, easily and quickly showing the app (or at least part of it) is back to the internal memory. I use this trick after each update session in order to quickly identify and transfer back to SD on both devices, as wished. This also makes it easy to identify newly installed apps eligible for it... This is of course not as good as if the apps would stay on SD after updates, but helps a lot as it makes it easy and quick to correct the problem. Hope such feature is or will be on later OS, such as on S10... my next mob.

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.