a safety issue?
no i doubt it
if a truck is getting hot enough to become a safety problem in such a way as to worry about it catching on fire? There are alot of things to consider before you blame the design of the truck.
I believe swoop hit on alot of it
Environment will be one of the first things evaluated if such a situation came into play...
The condition of the lifttruck and how it has been maintained.

Any design problems (if any) would probably be the lower down on the list of things to consider in a situation like that.

these kind of airtight engine compartments have been in the market for a long time, i believe some of the first ones i saw were with the yales 10 or 15yrs ago, give or take a couple years, and i'm sure there are other mfg's that designed similar ones back then. Sound abatement and sealing has been common since then.

In any case where i ran across an overheating issue due to poor air flow it was always a last effort solution but we would wind up installing extra vents in the hood either in the front or on the sides, depending on how the design of the hood would allow us to install them. I have only had to do this in a couple of occasions over the years of my working on lifttrucks though. And the foremost reason for the modification was because the truck was running in an extreme envrionmental situation like high dust or dirt or cotton etc.