"Errors detected on memory module" on 2019 Mac Pro 7,1

When error-correcting code RAM was first used, there was some controversy about whether to merely shift an [expensive] erring module to a location that would reduce the likelihood of a double-bit error. Today, that's nonsense.

A double-bit error causes a spectacular crash. Your data are precious, and you know that module is BAD. Replace it as soon as is practical. Don't wait.

User Tip: Mac Pro and Error Correcting Co… - Apple Community

Memory found to be bad At startup is walled off and not used, avoiding spectacular crashes.

However, on next startup, it may pass, and may be used, so you could yet see the very distinctive kernel panic machine check, caused by an uncorrectible error.

Older memories were not necessarily one entire word on one module. A word could be built form several modules, even several cards, to get a wide enough word.

Replacing part of a word might mean a half-rack of equipment swapped out, at huge expense. and substantial downtime.

Costs and difficulties of swapping out bad stuff are not nearly so high now.