Comment 1 for bug 1923370

AttachEffect.DiscardOnEntry=yes applies to objects entering transports, not moving underground. Subterranean movement is just another type of movement, the unit does not vanish from the map and doesn't enter anything, so the tag does not apply to it.

TrailerAnim= on an AttachEffect animation works properly only when the object is guaranteed to be always on the ground level. But on an AircraftType (and i think on a jumpjet VehicleType too), it will always detonate on the unit's "feet" (which means that if the unit is flying, it won't fall to the ground, but detonate inair). Thus, it doesn't surprise me that it also works erroneously for subterranean units.

If you need to use anything for AoE damage, use a weapon on the AttachEffect animation (which does work, i'm using it in several places).

Either way, subterranean behavior was not updated since TS, and thus, it works incorrectly even with vanilla RA2/YR logics (like radiation or open-topped). You'll probably have better results by simulating underground movement through unit-to-unit conversion than using actual underground movement.