While correct, I'm not sure that the information provided by MilFlyBoy is necessarily applicable to the DA-42, in terms of altitude limitation. The DA-42 is not equipped with an oxygen system, therefore it has no need to comply with 23.1447, so this would not form the basis of an altitude limitation. The powerplant restart limiting altitudes are lower than 18,000 feet, so these too are not the basis of a limitation, 'cause it's already exceeded.
23.1527 states that the maximum altitude must be "established" . It is 23.1583, which requires the maximum altitude be a limitation, but it still does not require a reason for the selected altitude for th elimitation, just that here must be one.
So I expect this returns to the fact that a manufacturer is not required to state why an altitude limitation is imposed. I can state that a Lycoming powered DA-42 is capable of flight at 19,500 feet, 'cause I've done it during testing, but I think that version of the aircraft retains the 18,000 limitation too. I know that in Canada, other diesel powered aircraft were altitude limited based on the engine operating characteristics.
Perhaps this limitation is more "visible" to the OP, as most other CAR 3 certified types were never required to have a maximum altitude, so they just did not express it a a limitation. That could appear to make the DA-42 stand out, but all Part 23 aircraft would have this limitation expressed.