You also have to expand all the sub-groupings and transition packs with a red dot and click on each one to clear them.
It seems like PD counts all content on your PC that isn't specifically included in PD19 as "new" the first time it's opened. It also seemed like a couple of the red dots showed up again one time after I had closed and reopended the app several times, but overall they seem to stay cleared on my two systems.