I have what I believe is engineered hardwood in my entryway. The first row of planks next to my entry way have rotted out. The threshold for the door was completely rotted as well. My entry door is surrounded by brick and upon closer inspection had zero silicone around it. I have since sealed it with silicone.
I have gone into the crawl space and pulled back the insulation to examine the subfloor in that area. I saw a little evidence of moisture but it did not appear there was damage.
Could water come in where the exterior casing was not sealed gotten caught between the flashing and the flooring causing the hardwood flooring to rot without damaging the subfloor?
It seems likely that if the visible flooring rotted, the subfloor rotted too; if nothing else because of simple proximity (one bad apple...). But I think you'll discover the answer when you take out the engineered wood!
Have you inspected the sweep (weatherstripping on bottom) on your door? It may be possible you have seepage through there during rains. over time the floor stays wet enough to rot out. Don't need alot of water to deteriorate it.