What kind of frame rate are you getting in Tunnel Hunt?
(F11 in MAME).
If it's very very low, that could explain things, because your coin might be activating the switch at a time when the game's code is not "looking" at the coin switch register.
See the way it works is that games have one screen refresh to do all their "logic" before it's time to redraw the screen again (which with arcade hardware, it's 60 times per second).
Now, programmers would try and get every millisecond they could if they were trying very "challenging" things for the particular hardware. Tunnel Hunt does alot of scaling and mock 3D yes? There's a chance that in order to gain extra coding milliseconds, the programmer(s) may have relagated looking at the coin reg maybe once every 30 frames, rather than every frame (in other words, rather than checking with every screen refresh, which would be 60 times a second, maybe it's 1 or 2 times a second).
To make a long story short, if such tricks were being done, it wouldn't affect the original hardware, running at its original frame rate of 60fps. But in an emulated version, running slower, with frames being dropped, perhaps the coin checking algorithm is sometimes being skipped!
This is all conjecture.

I'd also check the bug list in MAME.
~Ray B