Yes, I have the same s@!t situation, after I received the cable I realised that I have to solder. It is a cheap component I don’t want to spend time on those tiny parts. Just looking for another supplier. Never met with this issue with flex cable.
Alright, the consequence is: The aftermarket battery has so poor performance, all the screens was okay. The bottom of the screen is warm caused by backlight flash. solution in my case is: another new battery from different vendor
I asked my friend after I detected this same issue with an iPhone SE. He is an *original* LCD refurbisher from Hungary, and he said, yes this happens when you disconnect the screen from the phone. It will appear because the screen need to be “charged up”! So don’t worry about the flickering backlight