That's totally normal and, unfortunately, the way it is. In Android now, apps will only run their main file from Internal Memory, period (unless your phone supports adoptable storage, which basically 'bends the rules' in the phone and making it think the SD card is part of the internal memory, securely, of course). When you move an app to SD in phones that support that, you're actually only moving a part of the app, if at all. It's up to the dev to decide which parts, if any, are movable, and these are usually game saves, external downloads, media, etc.
OK, so what does that have to do with the stuff coming back to Internal Memory? Well, it's the way Android updates work in your version (a bit has changed on Nougat, but not a ton): Just like apps' main files must reside in internal memory, so must all application installation operations. So, every time you install a new app or an app gets updated, the phone has to carry out these operations in internal memory (which is also why it usually asks you to have 2.5 times the size of an app in free memory, so that it has room to expand the files, install, move, copy,etc.), and so, when you update an app that's got some of it in SD card, the system moves it back to internal memory for the update operation. Unfortunately, for security reasons (or so they claim), they're not moved back to SD card automatically.