I have the same issue. But I have the 6MT and it does it when slowing down to a stop and with the clutch out. I took it to the dealer and they were able to duplicate the "issue" but I was told that it is normal for the revs to drop. After they told me that I just learned to live with it. It doesn't bother me so much now. It's a "quirk" i have adapted to.
I have my DMV registration coming up soon so I will need to take the Legacy back to get the Recall WQC-39: ECM Reprogramming
taken care of.
The recall is for the ECM software logic. So I am wondering if there is a line of code written on the reprogram that will also alleviate this issue. Probably not but keeping my fingers crossed