Little update on this, Jens. I converted the A1200 to a hybrid PAL (preserving the NTSC system clock.). No behavioral changes to the issue.
I was able to acquire another A1200 and the card has been operating perfectly. I would gladly revisit this if your team has any ideas-or, even loan the board to your technical team for diagnostics. Clearly there is something defective with the A1200 that seems to have no impact on any other aspect of the system's performance-other than the ACA. (recall that it does not survive cold boot after flashing)
After spending some time with the ACA, it is an absolutely beautiful hardware upgrade. Just a shame we were not able to solve this specific issue.