VenkyPaiyaits a "hardware" issue. basicially there is a tape that grounds the air triggers which over time starts loosening due to heat build up, once the air triggers get loosend up and loose ground the triggers start to behave weird. This probably happened because the repair shop opnened your phone with high heat, so its 100% shop fault.
As requested I sent the details in personal inbox. Please let me know. Also I tried hard reset it didn't worked. Only left air trigger is flickering.