If we look at the original game, in almost all use cases of this object, it's buried in terrain. Usually, this terrain goes far enough that it fills in the gaps between the bottom of the object and the flames, even.
The only exceptions I can find are:
- Mayhem 2. In this case, the flames are used in a similar manner to water along the bottom of the level, and have no terrain under them in the first place - though they are placed high enough that their trigger area does not reach to the bottom of the level.
- Sunsoft 22. This isn't a true exception as such, but rather, two fire objects are placed above the terrain, with extra terrain in turn placed along the bottom of the fire objects. There's a third fire object that's directly buried in normal terrain.
(This includes Budget Amiga and Genesis levels. It does not include the PSP levels, as these were fan-remade by hand, so are not useful information here.)
Basically, the official games always bury it - of the above exceptions, one isn't really an exception as much as a "this could look like an exception if you don't look closely" case, and the other is a very unusual use case. Now of course - that doesn't necesserially outright serve as a reason to keep things as-is (after all - maybe they're like this *because* of the trigger area, although I'd have to wonder - at least for DOS / Amiga - why they didn't change the trigger area instead of designing around it, unless this was intentional), it's just a point to consider.