The iPhone 6s Plus doesn't have the problem. It is differently laid out and has a steel stiffener bar along the logic board. You shouldn't have a problem with it. You essentially got quite a bargain for a new phone and they gave you quite a lot for your "trade in". . . now if they will refund you an additional $200, you will be in good shape on that one. GRIN.
Some of us, including some who looked into the phones themselves, were theorizing that the real problem is the growth Tin Whiskers as the products aged caused by a new EPA requirement for non-lead containing solder.
Lead essentially retards the growth of Tin crystals in solder and taking the lead out removes that inhibition. The crystalline whiskers grow and start shorting out the very small traces on the logic boards. Bending the phone a bit breaks the short and it will work again for a bit. Then it shorts out again. Other electronics with the new solders are showing the same thing.
How the EPA was thinking the lead was going to escape the alloy to endanger anyone is any body's guess, but it is, as always, overkill to achieve unachievable goals with unintended consequences such as growing whiskers.
Apple started using a different solder alloy with other whisker inhibitors in it to replace the lead starting with the iPhone 6s plus and then the iPhone SE.
I don’t know how you know all that detail, but thanks! It makes perfect sense. Probably has to do with concerns about phone disposal (lead in the phone). Just a guess.
I hear now Apple may start making phones in the states — there was something linked on Drudge about it recently.