Just tried in a full tunnel again to make sure. Weirdly enough, with the BSP/imported mesh, it’s top-of-tunnel and bottom-of-tunnel motion that can potentially trigger this issue, but I can’t get it to trigger on purely left/right motion. Same case with both a complicated 4-hull collision mesh as with a Simplified 26DOP collision mesh.
EDIT: Put the plane on the outside of the tunnel and then strafed right into the outer side of the tunnel, and triggered a .4-X-direction collision. (This was after it had bounced far off of the tunnel wall, not just in a series of extremely quick grind-y collisions.) It’s so random…