Processing script returns no layer

Question

I am trying to run a processing script without a user defined layer output. The example below should create a vector layer with one point. The script runs without any error but the layertree is empty. If you run an other script afterwards, for example "Buffer" the layer is available and you can buffer the result from the script below.

What do I miss to make the created layer available for the layertree? My guess is that QgsProject.instance().addMapLayer(vl) does not work with the alg class? I know that the correct usage of the processing script would be using a QgsFeatureSink, but I am trying to do it the way below to get more control over the result layer.

from qgis.processing import alg
from qgis.PyQt.QtCore import QVariant
from qgis.core import QgsProject, QgsFeature, QgsField, QgsGeometry, QgsPointXY, QgsVectorLayer

@alg(name="test123", label=alg.tr("test123"), group="geophysics", group_label=alg.tr("Geophysics"))
@alg.output(type=alg.NUMBER, name="OUTPUT", label='Number of features processed')
def test123(instance, parameters, context, feedback, inputs):
    """
    tries to add a vectorlayer through a processing script
    """
    vl = QgsVectorLayer("Point", "Hello", "memory")
    pr = vl.dataProvider()
    pr.addAttributes([QgsField("name", QVariant.String),
              QgsField("field1",  QVariant.Int),
              QgsField("field2", QVariant.Double)])
    vl.updateFields() 

    f = QgsFeature()
    f.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(20,20)))
    f.setAttributes(["Hello QGIS", 100, 10.1])
    pr.addFeature(f)
    vl.updateExtents() 
    QgsProject.instance().addMapLayer(vl)
    if vl.isValid():
       QgsProject.instance().addMapLayer(vl)
       out = 1
    else:  
       out = 0 
    return {"OUTPUT":out}