This is normal Windows operation and is not a bug with the NAS. This is an explanation from Microsoft on the time stamps for files and folders.
Created: It is the time when the file is created in the
current directory. When the file is copied to a
new directory, a new value will be set.
Modified: It is the time when the file is last modified.
When the file is copied to elsewhere, the same
value will be carried over to the new directory.
Accessed: It is the time when the file is last accessed.
This value is set by the application program that
sets or revises the value. Unfortunately some
applications do not revise this value.
For a quick reference, refer to this table on when the timestamps change: