The difference is that a whereHas executes one query on the primary connection (like you see in your raw query)

But the with and withCount execute a second query on the connection if your Employee model.

As written in the documentation:

Eloquent does not currently support querying for relationship existence across databases. The relationships must exist within the same database.

A way to work around it is to set the $table property on your Model that includes your database name.

class Employee extends Model
{
    use HasFactory;

    protected $connection = "mysql_2";
    protected $table  = 'mysql_2.employees';

    public function corporate(): BelongsTo
    {
        return $this->belongsTo(Corporate::class);
    }
}

Or if you need more customization (like a dynamic database) you can overwrite the getTable function.

class Employee extends Model
{
    use HasFactory;

    protected $connection = "mysql_2";

    public function corporate(): BelongsTo
    {
        return $this->belongsTo(Corporate::class);
    }

    public function getTable()
    {
        // We ask the database name on the connection and prepare that for the table name with a . in between.
        return DB::connection($this->connection)->getDatabaseName() '.'. parent::getTable();
    }
}