" I find that 2147481343 seems to do the trick. This number doesn't make any sense to me. I suggest the customer try this number. The customer replies back: it works with google.com, but it does not work with other domains." My extrapolation: I find a potential fix: don't test it, don't understand it, and send it to customer. How is this acceptable?
Your extrapolation is incorrect. The following description matches the factual components of what you see as unacceptable, but includes the missing context from the post that makes the engineer's actions acceptable rather than unacceptable:
It was sent to the customer to collect experimental results while investigation continued in parallel. Per the post, production mitigations had already been put into place, and the customer was knowingly participating in an investigation of an unknown issue. The customer provided the requested experimental results.