Tech support called me before I saw the post. Wow! Nice service. We set up a plan and a follow-up call for today.
My configuration is a 2011 iMac with the non-beta softphone app installed from a USER/non-Admin account (i.e., for app installs you must specifically authenticate to allow it).
The last times I noticed the SIP credentials loss occurred was after a power-outage, and after applying the recent iMac firmware updates.
Here are the results from a series of tests:
1. Full shutdown, with and without the "reopen apps" block checked - ALL GOOD (no credential loss)
2. Restart with and without the "reopen apps" block checked - ALL GOOD
3. Powered off surge protector - ALL GOOD
Sooooooo, I have to say all is working properly. Barring some id-ten-t error I made, I can't think of other scenarios. I did just launch the app between tests and never actually used it. I'll go back make a call and see if that changes anything. For now, all is well.
A final note, from the tech I learned the app can properly display call logs and access things on the screen even with the SIP credentials lost or encountering the 302 error. The credentials are only required to make/receive calls.