Adding a output pin to a collapsed graph through dragging it on adds it to the list of inputs and doesn't update the input/output nodes.
Found in 4.19 CL# 4033788, 4.20 CL# 4302132, and 4.21 CL# 4370874
In the collapsed node details a Input will be added instead of an ouput, while being on the outputside of the node. Additionally the inside of the collapsed node will not be updated.
An output pin would be added correctly to the collapsed node and updated properly.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-64219 in the post.