iPhone stuck at logo after charging flex replacement.

Hi!

So I replaced the charging flex in my brothers iPhone 7 Plus, and I booted it up and it didn’t went beyond the apple logo, it just stays there. I tried to take the long connector, that’s sits on the charging flex and connects to the logic board, and pull that out and put the original charging flex connector in the slot, all this while the replacement flex was in the phone (yes I know it’s stupid) (see this YouTube video https://youtu.be/luyWpm8sr7U at 2:10 and you’ll know what I mean) and it worked.

so I installed the original charging flex back into the phone, and now the phone won’t boot with the original, apple logo appears and won’t go away (same issue), but I can put in the replacement cable connector into the logic board (just like the video) and it boots up fine.

I want to stress out, that I was extremely careful! I’ve studied the cables and connectors and none of them are snapped.

Any answers are appreciated! Thanks in advance!

edit: it was a grade A replacement flex I bought.

edit: the phone turns on fine when there’s no flex cable connected to the logic board at all.