Only because this policy is only implemented recently, at least from late 2006 onwards. Old 091 and 092 images don't show the white lines either. They only appeared in the refitted/improved Hans. It is likely the refitted Xia will have this too.
In the CDF, there is one picture that shows a Song actually having the white lines. Most Songs were built before this recent requirement. Another picture of a Song being built in the dock show that surface of the lower hull isn't even, that there is a slight bulge, which can be taken as indicative of a flank sonar. See where the red arrow is pointed at the picture. Papers, such as the SDF, also refer to the Song having a flank sonar. Flank sonars were also tested among Mings.
Please note that the Song pictured is actually one of the earlier ones, although the sail has been modified into the late version design.
Kw64, you probably didn't see the picture of the first Yuan in the dry dock with a young PLAN sailor posing in front of it. The sailor is a drummer for the ship's launching ceremony. Check those patches.