Explain the need for an initial bridge in a subflow starting with a channel

[garyrussell]

See https://stackoverflow.com/questions/55591704/whats-the-purpose-of-automatically-added-bridge-component-after-router

[artembilan]

See my comments in that SO question.
Maximum I would consider this as a documentation improvement for better end-user experience.

[garyrussell]

I am ok with closing this as "Works as Designed".

[garyrussell]

Converted to "documentation"