If you are using PPro and are rendering with GC on, can you please try to set the transmap gamma to custom (1.0)? I had your effect myself but could not reproduce it, to check if it is PPro trying to correct the transmap with CG where it should not do so. If you change the gamma it will effect all nodes where you are using the map (the custom gamma ist not stored per node, it somehow is stored as part of the bitmap).

If you are not using GC please ignore my post.