Transactions Committed After Lock Not Queryable?

Question

I have created a function that is:

• Run every hour in a batch job.
• May be called on demand.
• May be called on-demand by a webservice method.

It is summarised as follows:

• Lock the parent records.
• Query the child records.
• Process the child records to ensure that there are no duplicates.
• Write to database.

So, something along these lines:

    public static void runMethod(List<Id> ids) {
      List<MyObject__c> records = [SELECT Id FROM MyObject__c WHERE Id in :ids FOR UPDATE];
      List<ChildObj__c> childRecs = queryChildRecords(records);
      List<ChildObj__c> childRecsIns = checkDuplicatesAndCreateInsertList(childRecs);
      insert childRecsIns;
    }

If the hourly job (A) gets executed at the same time as the on-demand (B), the FOR UPDATE creates a lock and thus forces the 2nd process to wait until the first one commits. Hence, this protects against duplicates created in the ChildObj__c. I believe by doing it this way, it will only ever allow runMethod to run once per recordId.

However, from time-to-time, I occasionally get duplicates created and I cannot figure out why. I estimate it's maybe happening around 0.5% of the time and I'm unable to reproduce the issue. For me, it's as if when (A) finishes the transaction, the queryChildRecords in (B) occasionally does not pickup the latest records if queried immediately after. But to my understanding, this is not how databases work - however, I can't think of any other reason why the issue occurs.

Or another theory is that it also appears that the lock is occasionally released early before the commit and hence (B) is not retrieving the latest records. But to my understanding from Salesforce docs, this should not ever be the case.

I've also edited the post to indicate that this method can also be called by a webservice method. From my research, this shouldn't change anything. I've done testing to simulate this call (via POSTMAN) and the lock appears to be working as intended and I cannot re-produce the issue.

From the logs so far, it appears that either the batch job or the run on demand can create the duplicates.

Hypothetically, if I removed the FOR UPDATE in the SOQL, I can reproduce the duplicates 100% of the time, so I know that it's taking effect.

Appreciate any insight or assistance.