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Whatever you do, when you update apps they are moved back to internal storage so you lose the "free" internal memory.
/t5/Xperia-XZ/Related-to-application-saving-location/m-p/1179475/highlight/true#M2218
Hi @Yakk1, I'm sorry to hear about that. 
If you have a lot of application moved to the SD card and Google Play store is set to update them automatically you will have to manually move them again in Settings > Apps > name of the app > Storage > Change. Just like @alexdon mentioned.
Could you provide us with a screen shot of the Settings > Storage > Internal storage? So we can see what is filling up the internal storage in your phone.
You can also check in Settings > Apps if some of your installed applications are saving a lot of data on the internal storage, for example offline play lists, books or similar.
Keep us updated!