Things just got weirder. I just confirmed that the source of this issue is almost certainly NOT hardware based.
My BRZ Limited had not been resetting the IAM on restart, however recently (as in today) it started to reset it. I think I may have noticed a pattern that may explain it.
On my car, it seems that if the current IAM is less than the initial IAM of the rom you're running, then it does not reset. (IE - Was running Moto-easts tune, default IAM is .7, when mine was less than .7 it would not reset on restarts.)
When your stored IAM is greater than the default for that rom, it resets at startup. (IE - my IAM was at .74, restarted the car and since the initial IAM on this rom is .7, and .74 is greater than that, it reset to the default for that rom. Perhaps I wasn't seeing this behavior before since I was running another vendors tune who set the IAM to 1 by default, thus making it impossible to reset at restart if my theory is correct.)
Here's a screenshot of a datalog I did showing the IAM pre-startup, the startup sequence, and the IAM post startup:
