When I open a file that uses Altiverb, I find that Altiverb has returned to the default Impulse Response (IR). Other settings, such as Altiverb’s EQ, brightness, etc. are retained but the chosen Hall/Stadium/Room has been lost and has returned to the default IR.
 
Just to add to the mystery, this doesn’t always happen—it happens maybe 80% of the time, and I don’t know what the variables are. (In regard to variables, I almost always work the same way, using the same template and same routing).
 
There is almost nothing (at least that I could find) on the internet about this issue. I sent a tech support email to Audio Ease and the only suggestion was to turn off the automation read for the bus/track in which Altiverb is placed. This didn’t solve the problem.
Any suggestions?

Unfortunately I have no explanation/fix for you, but I have been experiencing the same thing for close to a year now - just so you know you aren't alone...
The only workaround I have is to take a screen shot of all my Altiverb instances in a project, and remember to check them each time I re-open... a real pain, but it becomes second nature after a while haha...

Thank you Biscuits for your response. It actually does make me feel better to know I'm not the only one experiencing the problem.
 
I guess Altiverb is not a popular plugin for Sonar users, as I would expect others to be chiming in saying that they have the issue also (or the dreaded "I don't have this issue, there must be something wrong with your setup"!)
 
My only 'workaround' is to type the IR I used for the project in Sonar's notes section.

I use Altiverb but have not had the problem