I ran into exactly the same situation doing the exact same repair on my son's series 6 Apple Watch. I didn't figure out what was going on with it until I got out my magnifying glass and looked very carefully at the display cable. It turns out that despite how careful I was in detaching it from the display, I still managed to damage it.
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The arrow points to the place where the flex cable got torn in the removal process. As it turns out, that cable is replaceable, but it does require some pretty good soldering skills - and a microscope would be very useful. I haven't gotten around to taking it in to work to borrow their excellent equipment yet, so I haven't finished the repair but I hope to in the next few days.