These two conditions sounds contradictory.
If the sd card would be used as internal memory Android 6+ treats it as one single logical memory block. No reason for differentiating any more. So you could not say if data is stored on the sd card or internal memory. That’s the way I understand this mechanism.
Also I believe you could not “copy” any app from/to sd card as logically it should be the same storage area.
What I can think of, for whatever reason it finally is not used as internal but external memory.
External memory should then be formated as exfat and if not encrypted accessible by a conventional computer OS using a sd card reader (maybe with adaptor).
Usually internal memory goes first before external memory. Since the card is full now you could try anyway to copy some apps from it to the internal memory.
If having used up every single bit it may help to free up little space to get the least file operation on your sd card going. Some data should be moved e.g. to a computer by usb.
Such things may still happen these days although filesystems has greatly improved meanwhile.
But as I have wrote, it sounds contradictory.
Another long way would be to backup your personal data to a computer and repeat the format procedure with the phone again assuring the card is properly integrated and used as internal memory. 15GB these days are not much too handle.
Then restore you data on the phone.
If you don’t want to reinstall and setup all your apps again a professional backup tool is needed. I am not into this on Android, but here are plenty of posts on such topics.