https://forum.unity.com/threads/polygon-collider-2d-seems-to-be-blocking-other-colliders.1472220/
TL;DR: Essentially, when the 2D polygon collider for my hex tile sprites is off, I can select my "City" 3D box collider mesh just fine, but when the collider is on for the tiles, the raycast seems to go straight through to the tile.