Controversy over the story of the crucifixion of a Canadian soldier during WWI

I didn't realize that there was a controversy over the story of a Canadian soldier being crucified by the Germans during the Second Battle of Ypres. This came to my attention when a historian reviewed my book, "The Great Promise". She questioned the journal entry because the no one could verify if it was rummer or factual. The following is a direct quote from my grandfather's journal:

At daylight we were at it again, the first thing that met my gaze was a shell dropped just the other side of the hedge, among what was left of a Canadian Battery Wagon Line, (most of the men had been killed when the Germans broke through the previous week they bayoneted them whilst they slept. Hung the Ferrier to a tree, and crucified a Sergeant of the Canadian Scottish to a barn door with bayonets. This wagon line had about a dozen horses left of 200 – the guns were captured by the enemy, but were afterwards regained by a magnificent charge by the Canadian infantry. These are fine fellows and splendid fighters and hated the cursed Germans like fury for their murderous ways of waging war. A couple of days previous the Canadian Scottish were ordered to retire, but refused to do so. [They] charged the enemy on their own, it was a mad thing to do and they lost over 500 men, but captured some trenches and captured 100 prisoners or more, not one of these prisoners were brought down. We were fighting as they – no quarter and the Canadians gave none. Just in rear of our guns, there was a Prussian Guardsman ( a fine fellow, fully 6’ 3” in height and big with it) pinned to a tree with a bayonet and a post card stuck on his forehead with the words, “Canada does not forget”. The by word of the Canadians were, “will give’em   crucify”. The happenings around of this period would fill a book with horrors of this description, and of the splendid fighting of the Canadians and the Indian troops who were with us.