"Require password for free downloads" option is not working properly

I have an annoying situation on iOS.

I've chosen the option to not type password when downloading a free item, but some time after (about a day), when I try do download free items, the password is required again. Then, checking the setting again, the option "require password" is enabled. Or worse, sometimes I check the setting and the option "require password" is disabled, and even so the password is required.

I don't know if it's a bug, or if it's working as planned by Apple, the fact is that it's too annoying!