It was explained to me by the certification authority that diesel engines have had difficulty demonstrating cold restart due to the glow ignition system being inadequate. I do no have first hand experience with this, as my DA-42 diesel flying was not for the engine testing, and my DA-42 Lycoming 360 flying had no problems with cold restart, I did dozens.
We did briefly fly the Lycoming powered DA-42 at 19000 feet on one engine (as the other one had quit due to fuel exhaustion of the hot fuel being tested). But, we did not actually try to maintain altitude up there, as we were headed down anyway. A DA-42 can maintain single engined flight at 18000 feet?
That said, I would expect that manufacturers, who are not marketing their aircraft for high altitude use, would rather just limit them by altitude, rather that going to the additional effort to demonstrate compliance way up there. I often negotiate with clients what they want to show compliance to, so as to not expend extra test effort for no perceived gain.