Tanaka, here are your answers.
1) The M18 had LOS to the top floor of the building occupied by enemy infantry. It could not trace LOS to the bottom floor during the turn before (I checked). I didn't want to issue hunt command to the M18 due to enemy armor nearby. I issue direct fire orders to fire the main gun at the enemy infantry.
2) Daytime, Clear Skies
3) I "guess" the gunner could miss. The trajectory from barrel is rather flat and straight at this range for a 76mm. The Gunner could not visibly see the enemy infantry moving down stairs. The gunner indirectly fired WITH a hill directly in front of him. If I was the gunner... I'd make sure I cleared that hill!
4) The M18 was stationary.
I'm not sure what parallax effect is.
There's a process with targeting, which might be causing this bug. The game checks LOS, then 'attacks.' Perhaps the game checked LOS, returned true, tracked the target (target is moving, thus requiring barrel suppression), then fires upon acquisition even though the shot might no longer be true. Ergo, the shot lands directly in the face of my M18 (which isn't good!)