How did you 'fix' the issue ?
Wipe SD card or factory reset or did you get to fix it at all ?
I did have loads of MP3 on my phone, but when space started to run low, I removed them all...but noticed it barely touched the free space. Must be some kind of caching going on when you use USB to copy stuff back and forth from your phone, but you would assume when you deleted the original content, that this would be removed too. Just speculation at this point...don't really know why this partition is eating so much space and what's contained within it.
I checked the SD card via FX explorer, but can't see the system data partition, so can't delete it.
I may have to wipe the SD card and see, but hate the hassle of setting up all my folders and favourite apps again