If you check "Let MobileSheets Manage my Files" then you should be able to uncheck "create subdirectories....." After that you could uncheck "Let MobileSheets Manage my Files" again, but i am not sure if that makes any difference.
Samsung Galaxy Tab S7 FE Android 12
Samsung Note Pro 12.2 LineageOS 14.1
Huawei Media Pad M3 lite Android 7
Samsung Note Pro 12.2 LineageOS 14.1
Huawei Media Pad M3 lite Android 7