I am attempting to match up users coming in from an external POST request to Salesforce users via a SOQL query of the name that returns a user record, then use the ID from the user record to insert a custom object record with a User Lookup field. However I am getting the following error: 

Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, User: id value of incorrect type: 005U0000003GvJKIA0: [User__c]

This error is being thrown because the ID needs to be 15 characters, but is, in practice, coming out as 18 characters. I have tried a few different methods of limiting the string to 15 chars, but to no avail. This is frustrating because it works just fine in the sandbox environment, it is only when deployed to production that this error comes up.

Methods I have tried:

1. No string correction

// match names list to users and create records for each user
        finalCount = names.size();
        for (Integer j = 0; j < finalCount; j++) {
        
            User matchUsr = [SELECT Id, Name
                                FROM User
                                WHERE Name = :names[j]
                                LIMIT 1];
            String usrId = matchUsr.Id;
            
            indivs.add(new Late_Tasks_by_User__c(User__c=usrId,Dependent_Tasks__c=deps[j],Actual_Late_Tasks__c=lates[j],Date__c=datej));
            usrId = '';
            echoOut += names[j] + ', ';
        }

2. Substring the usrId string to (0,15) 

// match names list to users and create records for each user
        finalCount = names.size();
        for (Integer j = 0; j < finalCount; j++) {
        
            User matchUsr = [SELECT Id, Name
                                FROM User
                                WHERE Name = :names[j]
                                LIMIT 1];
            String userMatch = matchUsr.Id;
            String usrId = userMatch.subString(0, 15);
            
            indivs.add(new Late_Tasks_by_User__c(User__c=usrId,Dependent_Tasks__c=deps[j],Actual_Late_Tasks__c=lates[j],Date__c=datej));
            usrId = '';
            echoOut += names[j] + ', ';
        }

3. Take one character from the ID string of the user record at a time (0-14), and add to new usrId string

// match names list to users and create records for each user
        finalCount = names.size();
        for (Integer j = 0; j < finalCount; j++) {
        
            User matchUsr = [SELECT Id, Name
                                FROM User
                                WHERE Name = :names[j]
                                LIMIT 1];
            String userMatch = matchUsr.Id;
            String usrId = '';
            for (Integer xc = 0; xc < 15; xc++) {        
                usrId += userMatch.subString(xc, xc+1);
            }
            
            indivs.add(new Late_Tasks_by_User__c(User__c=usrId,Dependent_Tasks__c=deps[j],Actual_Late_Tasks__c=lates[j],Date__c=datej));
            usrId = '';
            echoOut += names[j] + ', ';
        }

None of these have produced a working result. All three continue to return an 18 character string as the usrId variable. I have found several threads on this forum regarding this topic, but the only solutions that are listed with those questions are either syntax error with the specific code, or someone saying that you have to subString the ID. If anyone has any solutions or ideas beyond that, please let me know.

Thanks,
Joe