There's a rumor that older AGESA[1] versions had a bug that prevented the chipset from recognizing and utilizing ECC ram properly even though the chipsets should support it. Check any motherboard in question for a firmware update which includes at least AGESA 1.0.0.5 patch C.

https://www.reddit.com/r/truenas/comments/10lqofy/

[1] Part of the firmware on AMD systems, that brings up core system components. https://en.wikipedia.org/wiki/AGESA