I'm seeing this very same thing. In one org, my test covers 100% of the target Batchable class. In another org, the very same code in both the test and target classes does NOT exercise the .execute() method of the Batchable class.

The one that works is a production org. The one that does not is an unrelated sandbox org.

Here's the code just FYI

global class MV_testBatchable implements Database.Batchable<sObject>
{
    global final String query;
    
    global MV_testBatchable(String q)
    {
        query = q;
    }
    
    global Database.QueryLocator start(Database.BatchableContext BC)
    {
        return Database.getQueryLocator(query);
    }
    
    global void execute(Database.BatchableContext BC, List<sObject> scope)
    {
        List<Contact> contacts = (List<Contact>) scope;
                System.debug('YYZ - scopeSize='+scope.size());
                System.debug('RUNNING BATCH EXECUTE');
        for (Contact c : contacts) System.debug('YYZ - '+c.name);
    }
    
    global void finish(Database.BatchableContext BC){}
}

@isTest
private class MV_testBatchableTest 
{
    
    static testMethod void myUnitTest() 
    {
        String q = 'SELECT Id, Name FROM Contact LIMIT 10';
        
        Test.StartTest();
        MV_testBatchable batcher = new MV_testBatchable(q);
        Id batchprocessid = Database.executeBatch(batcher, 10);
        Test.StopTest();
        
        String query = q;
        List<Contact> result = Database.query(query);
        System.debug('YYZ - querySize='+result.size());
        System.assert(result.size()>0);
        System.assert(checkMe(result));
    }
    
    static boolean checkMe(List<Contact> cs)
    {
        Contact c = cs.get(0);
        System.debug('YYZ - c:'+c);
        return true;
    }
}