I tested your example, it works fine.
Then I tried to reproduce my example. I can reproduce the issue. Here is how to do it:
You must use UNC path. You can't use c:\ - has to be on the network. Try this example:
1. Create a variable "path" for \\server\folder\aaaa\parentfolder\
2. Set File Filter Location to {JOB(Active,Variable,path)}subfolder\ (When this is evaluated, it = \\server\folder\aaaa\parentfolder\subfolder\)
3. Set Decrypt custom destination folder to {JOB(Active,Variable,path)}
4. Put the encrypted files in \\server\folder\aaaa\parentfolder\subfolder\
5. Run the task
6. Look at parentfolder for the files. You will see a folder structure that matches {JOB(Active,Variable,path)}subfolder\ inside the parentfolder.