iPhone 6 backlight fixed but dead again

I agree with Richard, it seems to me that the repair shop should at least take a good look to understand why the phone has a similar failure. It may be unrelated but it should be properly diagnosed.

That said, what struck me in your question was that you paid 50$ for this repair. That seems a little too inexpensive to me. While everyone is free to charge what they want, consumers are often attracted by low prices and don't consider quality workmanship and good warranties. Micro-soldering is not exactly rocket science but it is still an acquired skill and doing repairs on high density logic boards, like iPhones, requires a gentle touch more than anything else.

Seeing shops race to the gutter on micro-soldering fees is not a good thing IMO. It devalues the work performed and encourages shortcuts and "quick" work as opposed to good work.