I don't really count the fall of the Roman empire as scientific regress.

I didn't talk about the fall of Rome but the decline of Roman civilization for a reason, it was a process that took centuries. Before it people could build things like the Antikythera mechanism afterwards they couldn't for what seems to be at least a thousand years. Before it literacy was more widespread than afterwards. Even if we didn't lose any scientific knowledge in the process, something I doubt because of the large number of lost works we know existed from references in the preserved ones, it certainly was known by fewer people.

The sack of Rome didn't (or at least shouldn't) cause farmers in the middle of nowhere to change their beliefs about the moral correctness of beating their slaves in particular circumstances. Changing circumstances != changing moral beliefs.

Wait what?

Of course moral beliefs in humans are affected by cricumstances! We have empirical evidence of this even in lab conditions. Changed economic circumstances seem to have am obviously big impact on social standards of morality as well.

Assuming moral realism, the human mind may not be designed to discover truth about morality any more than it is to discover turth about any other aspect of nature, it is designed to be as adaptive as possible (unless we are assuing a created mind for this argument as well). Also human minds just get things plain wrong due to limtied resources too.

so there every reason to think that institutions would fail at preserving knowledge.

Why do you assume we are good at using the scientific method to discover moral truth? If moral realism was true, then looking at moral change in the past few centuries it looks much more like the changes in naturalist knowledge we saw before the scientific revolution than after it.