FIXED IT! Skip to the end for solution.
Before ever seeing this thread I opened one of my sensors just to see what was inside. Apparently I jostled the battery loose for just a second and the sensor bricked as so many of you have described here. I read all these posts, tried shorting the battery terminals, tried leaving the battery out for a while… nothing worked. At this point I had already uninstalled the sensor from the bridge and I was trying to reinstall it. I just kept getting the infamous two flashes when I put the battery in. Every 10 seconds it would flash twice. I chatted with support and they are sending me a new sensor. The sensor was bought as part of a starter kit so I asked him if I was going to have the same problem with the other sensor. He said, just change the battery before it goes dead and you should be fine. I wasn’t exactly thrilled with that answer, but oh well. I had already wasted a great deal of time on this (as everyone else in this thread has probably done) but I decided to try one last desperate thing…
WHAT I DID:
I wildly and very randomly pulled out the battery and put it back in. Every time I got the two flashes until ONE TIME (after doing this about 10 to 12 times) I got just ONE FLASH (with NO further flashing every 10 sec like before). At that point I tried reinstalling the sensor from the app and it worked! I think it somehow caught the chip in just the right part of its powerup/powerdown cycle to force a POR. Who knows? Anyway, it felt good!!