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It's from the perspective of not knowing anything about the issue. It would look like jobs failing randomly one day when everything was fine the day before. Not hard to understand.

They've meant something like "arbitrary", in its "without any good/justifiable reason" sense. The word "random" is also used in this sense, especially when talking about human-made decisions.

Unexpectedly is probably what they meant

IMHO, what they said means that on day 65 all jobs work, on day 66, jobs work or don't, seemingly at random.

But what they seem to be indicating is that all jobs fail on day 66. There's no randomness in evidence.

Seems quite predictable given the others in the bug report encountering the same.