HiFiBerry DAC + ADC, U2 IC spontaneous combustion

We recently purchased four HifiBerry DAC+ADC Pro HW 1.0.1 and we just received them just 3 days ago.  We connect one of the boards to a Raspberry Pi 4 and ran a recording with a microphone connected via the 3.5 mm audio jack over the weekend to do some sound recording.  The unit ran fine with no issue continuously for 3 days.  All of a sudden this morning, the unit just burst into smoke while we were in the room just looking at it.  We didn't do anything to the unit besides programming on the Pi via the SSH terminal for several hours before we had a meeting to talk about the unit.  The unit was inside a metal enclosure designed for Pi4 and HifiBerry, specifically from HiFiBerry (https://www.hifiberry.com/shop/cases/steel-case-for-hifiberry-dac-pi-4/). We immediately unplug the unit and took the case apart to see what was burning.  The RPi 4 was fine since it boots up with no issue after we detached the HiFiBerry DAC + ADC hat from it.  We could see the burnt component on the HiFiBerry board labeled "U2".

We are not sure what happened since we hadn't had any issue like this before on 10+ other HiFiBerry DAC + ADC we bought from before.  Those were even deployed for sound recording for several months in the field.

The only difference I can think of is that we use the metal enclosure instead of the custom electronics polycarbonate enclosure. Another difference is that the RPi4 now has copper heatsinks on the ICs while the old ones didn't. Maybe the HiFi Berry Board somehow touched the copper heatsink on the bottom side, but I'm not sure how that's possible.  There's at least a centimeter gap between the heatsink and the HiFiBerry board and under areas with no protruding pins.  There's no sign of damage on the underside of the board.  We did correctly mount the HiFiBerry board with the gold spacer on the lid side, so the board shouldn't be drooping down.  The picture below shows what it looks like when it's sitting on top of the RPi 4.



We are a bit concerned since we are using the recommended authentic 5.25V-4A power supply from the iUNiKer kit (https://www.amazon.com/Raspberry-Supply-iUniker-Switch-Listed/dp/B097P2NLVH).  It seems to have a good rating despite the flaw of not being a true Power USB C with power negotiation.  It probably doesn't matter on the RPi4.  

From this forum, I gathered that U2 is an ADP151AUJZ-3.3-R7, a 3.3V regulator with a maximum of 200 mA output and can handle up to 5.5V input.  Unless there's a wild swing on the power supply output above 5.5V, I can't see the IC getting fried.  We didn't do anything with the HifiBerry board that would make it draw too much current.

In any case, we are a bit concerned since a spontaneous failure with no good explanation is problematic for our long-term unit deployment.  We are about to deploy 4 units overseas for a year-long so we want to understand what could possibly lead to this problem.

Thanks,
Patrick