replaced_by leaves the replaced port installed and says it cannot uninstall it

[ryandesign (Ryan Carsten Schmidt)]

The new replaced_by keyword doesn't always seem to work completely. In this case, when I asked port to upgrade libmp4v2, it correctly installed mp4v2 as its replacement, but then left libmp4v2 installed (but deactivated) stating faac depended on it. port deps faac shows it depends on mp4v2 not libmp4v2, though faac may have depended on libmp4v2 at the time that I installed it.

$ libmp4v2
--->  Computing dependencies for mp4v2
--->  Fetching mp4v2
--->  Verifying checksum(s) for mp4v2
--->  Extracting mp4v2
--->  Configuring mp4v2
--->  Building mp4v2
--->  Staging mp4v2 into destroot
--->  Deactivating libmp4v2 @1.5.0.1_1
--->  Computing dependencies for mp4v2
--->  Installing mp4v2 @1.9.1_1
--->  Activating mp4v2 @1.9.1_1
--->  Cleaning mp4v2
--->  Unable to uninstall libmp4v2 1.5.0.1_1, the following ports depend on it:
--->  	faac

$ port installed libmp4v2 mp4v2
The following ports are currently installed:
  libmp4v2 @1.5.0.1_1
  mp4v2 @1.9.1_1 (active)

$ port deps faac
Full Name: faac @1.28
Build Dependencies:   libtool, autoconf, automake, autoconf, automake, libtool
Library Dependencies: mp4v2