Hello Sam.
As you noticed, there are gameplay checks to make sure a player cannot get under the terrain. You will encounter this type of issue when designing tunnels and playing with the visibility tools.
Fortunately you can get around this issue by adding a “FortUndergroundVolume” in your map. This volume should encompass all the path the vehicle or player will take under the terrain (ie in this case the tunnel), this way, they would be exempt from checks.
Cheers