In Pipeline the panel() method of first stage is run twice

[rafgonsi]

ALL software version info

panel 0.10.0, bokeh 2.2.2, Python 3.7
OS: Manjaro 20.1.2

Description of expected behavior and the observed behavior

When using panel.pipeline.Pipeline sometimes the panel() method of the first stage of pipeline is called twice.

Complete, minimal, self-contained example code that reproduces the issue

import param
import panel as pn


class A(param.Parameterized):
    def panel(self):
        print("A.panel()")
        return self


class B(param.Parameterized):
    def panel(self):
        print("B.panel()")
        return self


pipeline = pn.pipeline.Pipeline(
    stages=[('A', A()), ('B', B())]
)
pn.Column(pipeline.buttons, pipeline.stage).servable()

After running this with panel serve bug_pipeline.py and opening in the browser, you see in console that 'A.panel()' was printed twice:

On the other hand if you declare stages differently, then the panel() method is run only once, as expected:

pipeline = pn.pipeline.Pipeline()
pipeline.add_stage('A', A())
pipeline.add_stage('B', B())
pn.Column(pipeline.buttons, pipeline.stage).servable()