In HELP-72829 validation of a fixed snapshot fails nondeterministically.

Results here: Atlas Search UI

The specific collection/records that fail validation on any particular node varies too.

Notably, in all failures, the invalidity (which can be missing or extra keys) looks like this:

          {
            "idKey": {
              "_id": null
            },
            "indexKey": {
              "Projects.AmountForTech": null
            },
            "indexName": "Projects.AmountForTech_1",
            "recordId": 1004
          }

The null _id key is likely SERVER-87215, whilst this is a problem (and possibly, related to the real problem) it is not the cause of this validation failure.

Far more likely, the null indexKey is the problem. When the validation is run with logdiagnostics=true we can see all of the index keys generated, none of them are null. So there has to be a problem to do with copying/moving/storing them, see comments on the linked HELP ticket.