So I noticed something about this, and it's disturbing because it's kinda out of control. Here's what I've found:
So apparently the com.apple.quarantine xattribute is set for files created with Sandboxed applications and downloaded from Internet, which we know. Removing the attribute solves the problem, where opening the file in Finder will actually set its last opened metadata. But why? It also seems that _some_ quarantine values still allow the metadata to be set.
Anybody know what the actual data in the attribute means?
For example, if the first 4 digits are something like 0041 or 0042, the last opened date will update. If it's something else like 0001, it won't.