Johnston may have been carried to a field hospital after his death, but he died in the ravine shown below. Perhaps other soldiers there thought he was still living.

Here's the account of Johnston's death from the National Military Park at Shiloh:

http://www.civilwarlandscapes.org/cwla/states/tn/sh/places/1jd04.htm

For those who may be interested, this is Shelby Foote's account of Johnston's death at Shiloh:

At this end of the battle line, on the far flank of the Hornets Nest, there was a ten-acre peach orchard in full bloom. Hurlbut had a heavy line of infantry posted among the trees, supported by guns whose smoke lazed and swirled up through the branches sheathed in pink, and a bright rain of petals fell fluttering like confetti in the sunlight as bullets clipped the blossoms overhead. Arriving just after one of Breckinridge's brigades had recoiled from a charge against the orchard, Johnston saw that the officers were having trouble getting the troops in line to go forward again. "Men! they are stubborn; we must use the bayonet," he told them. To emphasize his meaning he rode among them and touched the points of their bayonets with the tin cup. "These must do the work," he said. When the line had formed, the soldiers were still hesitant to reenter the smoky uproar. So Johnston did what he had been doing all that morning, all along the line of battle. Riding front and center, he stood in the stirrups, removed his hat, and called back over his shoulder: "I will lead you! " As he touched his spurs to the flanks of his horse, the men surged forward, charging with him into the sheet of flame which blazed to meet them there among the blossoms letting fall their bright pink rain.

This time the charge was not repulsed; Hurlbut's troops gave way, abandoning the orchard to the cheering men in gray. Johnston came riding back, a smile on his lips, his teeth flashing white beneath his mustache. There were rips and tears in his uniform and one bootsole had been cut nearly in half by a minie bullet. He shook his foot so the dangling leather flapped. "They didn't trip me up that time," he said, laughing. His battle blood was up; his eyes were shining. Presently, however, as the general sat watching his soldiers celebrate their capture of the orchard and its guns, Governor Isham Harris of Tennessee, who had volunteered to serve as his aide during the battle, saw him reel in the saddle.

"General -- are you hurt?" he cried.

"Yes, and I fear seriously," Johnston said.

None of the rest of his staff was there, the general having sent them off on various missions. Riding with one arm across Johnston's shoulders to prevent his falling, Harris guided the bay into a nearby ravine, where he eased the pale commander to the ground and began unfastening his clothes in an attempt to find the wound. He had no luck until he noticed the right boot full of blood, and then he found it: a neat hole drilled just above the hollow of the knee, marking where the femoral artery had been severed. This called for a knowledge of tourniquets, but the governor knew nothing of such things. The man who knew most about them, Johnston's staff physician, had been ordered by the general to attend to a group of Federal wounded he encountered on his way to the far right. When the doctor protested, Johnston cut him off: "These men were our enemies a moment ago. They are our prisoners now. Take care of them." So Harris alone was left to do what he could to staunch the bright red flow of blood.

He could do little. Brandy might help, he thought, but when he poured some into the hurt man's mouth it ran back out again. Presently a colonel, Johnston's chief of staff, came hurrying into the ravine. But he could do nothing either. He knelt down facing the general. "Johnston, do you know me? Johnston, do you know me?" he kept asking, over and over, nudging the general's shoulder as he spoke.

But Johnston did not know him. Johnston was dead.