Adopted external sd card as internal storage - but storage space is not available On my Xperia XZs with Android 8.0 I formatted the external SD card to be used as internal or adoptable storage. I had to use ADB to get this working. The system sees the storage, but I can't use it. 

Total Commander only sees the internal memory, which is 20GB of 32GB internal memory. The 128GB external SD shows up as 256GB of which half is used. 

How can I get the SD card showing up as internal storage?
First of all, open SD card from screen displayed on screenshot in your question, than tap and hold recents button in nav bar or tap 3 dot menu if it's displayed, you will get various options one of them is migrate data tap on that and it should be fine.
Now because total commander see only one storage it is good, it is same case on my phone, and it works fine. Also adopted storage doesn't appear as internal storage it stays under same name and as external storage, but thing is that you can open app options in app manger in settings and choose to move to external storage in that case app will end up on adopted SD card.
When did you adopted SD card because it's already half full and as you said it didn't work as intended, when SD card is formatted as adoptable all data from SD card should be deleted.
Since it's showing 256 instead of 128 you should repeat process of adopting since something went wrong.