I've seen the frame machine get stuck too close to the tree, and the tree refused to grow until the machine moves far enough away. Also, it's way too big to be entirely overwritten (including the wireless receivers on the back) by a spruce tree. Those floating logs? I reckon the Block Breakers disappeared first, then the frames got stuck to the logs and pulled them back, and then the rest of it disappeared.
I'll check where the chunk boundary is tonight to see if it's a chunk edge problem but I'm sure the area is thoroughly chunkloaded with ChickenChunks.
And yes, it's a tree harvester so it only needs to go back and forth three steps, and it's shaped like a spruce tree because it harvests spruce trees.