Is it a certification requirement that an engine can be restarted at hight altitude?
Why don't they say it in the AFM?
The requirement is here:
FAR 23.903: ......
(d) Starting and stopping (piston engine).
(1) The design of the installation must be such that risk of fire or mechanical damage to the engine or airplane, as a result of starting the engine in any conditions in which starting is to be permitted, is reduced to a minimum. Any techniques and associated limitations for engine starting must be established and included in the Airplane Flight Manual, approved manual material, or applicable operating placards. Means must be provided for--
(i) Restarting any engine of a multiengine airplane in flight, .... my bold
There are many examples of a design requirement being complied with by flight manual limitation, without the underlying reason for that limitation being explained in the flight manual. For this reason, unquestioning compliance with flight manual limitations is wise, as there may be an angle you've failed to consider, as to why exceeding a limitation is unwise.
That all said, it is speculation on my part about the starting limitations of the diesels in the DA-42, All my engine testing experience in the DA-42 was the Lycomings. My diesel experience in the DA-42, was to take it so the mechanic could buy some Mercedes engine parts to get it running properly again!