codeql not tracking certian object literals

When I use taint tracking to test code below

```javascript
function Compiler(source){
this.pp = source.pretty;
sink1(this.pp)
}

Compiler.prototype = {
compile: function (){
sink2(this.pp);
}
}

function generateCode(source) {
return new Compiler(source).compile();
}
var data = easySource();

generateCode({pretty: data.pretty})
```

```codeql
/**
* @name data-flow
* @kind path-problem
* @problem.severity warning
* @tags security
* @id js
*/

import javascript
import DataFlow::PathGraph

DataFlow::SourceNode easySourceTracker(DataFlow::TypeTracker t) {
t.start() and
result instanceof MyTestSource
or
exists(DataFlow::TypeTracker t2 | result = easySourceTracker(t2).track(t2, t))
}

class MyTestSource extends DataFlow::Node {
MyTestSource(){
exists(CallExpr call |
call.getCalleeName()="easySource" |
this.asExpr() = call
)
}
}

class MyTestSink extends DataFlow::Node {
MyTestSink(){
exists(DataFlow::InvokeNode call |
call.getCalleeName().indexOf("sink") != -1 |
this = call.getAnArgument()
)
}
}

class TestConfiguration extends TaintTracking::Configuration {
TestConfiguration() { this = "TestConfiguration" }

override predicate isSource(DataFlow::Node source) {
source = easySourceTracker(_)
}

override predicate isSink(DataFlow::Node sink) {
sink instanceof MyTestSink
}
}

from TestConfiguration cfg, DataFlow::PathNode source, DataFlow::PathNode sink, DataFlow::InvokeNode call
where cfg.hasFlowPath(source, sink)
and call.getAnArgument() = sink.getNode()
select sink.getNode(), source, sink, call.getCalleeName()
```

I found codeql can not find the taint flow to `sink2(this.pp);`
But if I modify `generateCode({pretty: data.pretty})` to `generateCode(data)`, codeql now can find `sink2(this.pp);`.

Although I found we could add `localFieldStep` to record all step from `pred` to `succ` through a field accessed through `this` in a class.
```codeql
override predicate isAdditionalTaintStep(DataFlow::Node pred, DataFlow::Node succ) {
DataFlow::localFieldStep(pred, succ)
}
```

But I am confused, why can codeql handled this case after modify `generateCode({pretty: data.pretty})` to `generateCode(data)` or why it can’t be handled before the modification.