No way is this strictly a hardware issue. If someone got a defective phone, the chances of them getting a second, third, then fourth defective phone would be so infinitesimally small. This has to be a problem with an app, or possibly a combination of apps.
By the way, my 128gb iPhone 6+ has never demonstrated this problem, but I only have 198 apps. (too lazy to discard unused apps) I'm not saying the problem doesn't exist, just sharing personal experience.