There's another passage in that book that this article might help explain: IIRC, there came a time when Henshaw and his fellow test pilots experienced some life

There's another passage in that book that this article might help explain: IIRC, there came a time when Henshaw and his fellow test pilots experienced some life-threatening engine failures of the Spitfires that they were testing, at one point forcing him to make a crash landing in an urban area (perhaps the Wednesfield crash of 7/18/1942 mentioned in his Wikipedia entry? And is that perhaps the incident mentioned in the last paragraph of your quote?) The cause was traced to something like a small change in the order in which the engines were assembled. That the British-built engines were hand-crafted seems to make this less surprising. I would guess that, being hand-built, it would be trickier to keep them well-maintained, and require a particularly well-trained and skilled corps of artificers. The article does not get into it, but I wonder if there were differences in reliability between the Packard- and Rolls-Royce- built Merlins?