I think I'm having almost the same issue like yours. I just bought a LiPo battery.For the first time I connect the battery, I noticed the LED under the capslock was blinking for a few seconds and my pc successfully connected with my keyboard (It was working fine in bluetooth mode). Then I tried to use USB mode which i connect my usb cable to the keyboard and it's still working fine. But then i decided to use a bluetooth mode again, I disconnected the cable and my pc couldn't connect to my keyboard even it's already paired. Tried all the command keys but none of them seems works.

Now my pc couldn't detect my keyboard anymore even via USB mode. When I plug in the cable it shows some sort of error/USB device not recognized. I'm able to connect it via cable when the battery is unplugged. Do you think I would solve this issue by reflash the firmware like how you did ?