In the case of more-than-one-hop-long promise forwarding chains,
it was possible to lose some callbacks. See the test case.
My brain almost melted before I discovered that, well,
blackbird "accidentally a callback".
Holy shit, I knew it!! I have been trying to track that down for years now, but every time I reduced the test case it magically started working again. Thanks, Ivan!
In the case of more-than-one-hop-long promise forwarding chains, it was possible to lose some callbacks. See the test case. My brain almost melted before I discovered that, well, blackbird "accidentally a callback".