I'm having the reverse of the problem everybody is having with their internal storage management. So I have an SD card in my phone (xiaomi mi a1, android pie) which was never set to be internal/app storage or anything similar. I have plenty of internal space in my phone by default, still 30+ GB free, and I'm using a 16GB SD card, on which I have about 13GB music. Today I noticed my SD card is full. It contains an Android folder in which there are like a hundred folders for every app I use and its exactly 3 GB.
Now, I never set up my SD card as app storage, even if I go to options->storage, it shows up as a portable storage. And checking some of my biggest apps at settings->apps->app info, cannot find anything marked with external storage or SD card, everything appears to be on my internal storage.
Also, it is probably not cacheing, because running google's cache freeing stuff, I don't experience any change in my SD card.