My earlier wording was the problem here, not your reading of it.
There are two official descriptions and they do not say the same thing. The CLI reference calls start-delay "the delay to wait before a call to start up the VM returns", which is the one I quoted. The XenAPI field description calls it "the delay to wait before proceeding to the next order in the startup sequence". That second one is exactly what you saw: the delay lands on the next VM to start, not on the one you set it on. I quoted the confusing one and then said it explained your behaviour, which it does not really.
What I still cannot explain is why a start-delay would stop a VM autostarting altogether. Neither wording predicts that, so I would rather say I do not know than invent a reason.
On auto_poweron_delay, I went looking and could not find it anywhere in the XCP-ng or XO docs, so I think your instinct about that search result was right.
I have updated the docs PR I had open so it leads with the XenAPI wording and cites both, since the CLI reference phrasing is what sent me wrong in the first place.
