FWIW, I recently took an old Sonar 7 project, opened it in Plat and did some Melodyne editing on a vocal track. I had no issues with unrendered clips not playing, and in fact I was quite surprised how well the old project 'worked' in Platinum.
Sounds like you've got one of those random quirky things that just happen! Having said that, the good news is that it sounds like you can narrow it down to a point in time when it started ie the Dorchester update.
Maybe you could try rolling back versions to see if the same projects play properly.