Probably not a bug. Those 12s have probably not learned to drive, and therefore, cannot drive their own vehicles even from the NPC motor pool.
You are correct. I added driving skill to a babysitter and invited it over. It went home in a jalopy. I remember seeing them driving in the past but that was before Generations added the driving skill.
On another issue, when HighFidelityPlantGrowth is enabled (for advancing inactive lots I assume), havestables in community garden remain static at 5:00 am. I had to disable this to see any changes to my garden.
One curiosity question. Did you change the chair finder protocol when Pets came out? Before Pets, my sims would go to a random chair in the same room as the bookshelf in the library. Now, they always go to the closest one.
Anyway, thanks for fixing the motor pool.