TaifunFile can write into /Download, 
but not into /Documents, see here API 30 workarounds v2 - #21 by Taifun
File component can write into /Download, but currently not into /Documents, see the bug report here Test "copy file from assets to shared directory and read it from there" in ai-test - #6 by Taifun
my guess is, you are testing the file component... 
which means, writing in to /Download even into a subfolder works fine...
Taifun