No, I didn't succeed to identify any cause to that, any log/warning/error either in log files or on standard output, no blocked request on host, no discarded message on Huginn side. At this point I have zero clue of what happened, I simply copied the URL, deleted the webhook, re-created it and pasted the URL again.
If I had to take a guess, I would say that somehow, the initially created webhook was not saved/applied. I know it is supposed to autosave, but I can't see any element that can confirm that the webhook was indeed properly set.
I also spinned up a new container in order to try and replicate the problem : I remember messing around with the dropdown list a few times and also clicking multiple times on a few UI elements when trying to configure the webhook for the first time. However, I was not able to reproduce the problem. Not much I can give you here... If I encounter anything useful, I'll keep you updated !