What could be causing this issue. With regards to application, the Arduino is connected to 6 current clamps through audio jacks. Currently a DC 12V 3A max power supply is being used which I fear is causing the issue. This was employed since previously the device was restating when the GMS chip was booting due to excess power draw.
I don't have a MKR 1400 GSM but a quick look on the official Arduino website seems to indicate that the supply voltage should be 5V at 2A (or more). I can't see any reference to a higher DC voltages as a power source.
EDIT: Re the booting issue, the Arduino website says:
Communication over cellular networks, requires current peaks above the maximum a USB port can provide. While the port reaches 500mA, a typical GSM handshake (when the board boots and registers on a network provider) could easily reach a 2A peak. Therefore it is recommended to either provide a source with a higher current limit over Vin, or having a LiPo battery of, at least, 2500 mAh.
I have tried using various power supplies (for the project batteries are not an option) and using a 5V 2A power supply the Arduino re-booted on GSM. It was only after increasing the supply to 12V 3A that the device worked reliably.
Admittedly, I'm only looking at the MKR 1400 GSM schematics here, but I can only see a 2-pin LiPo battery connector and a micro USB connector (and the antenna connector too). The LiPo connector is for the battery (probably around 3.7V) and the USB connector shows +5V.
There's also the VIN pin. The FAQ says that:
Vin: This pin can be used to power the board with a regulated 5V source. If the power is fed through this pin, the USB power source is disconnected. This is the only way you can supply 5v (range is 5V to maximum 6V) to the board not using USB. This pin is an INPUT.
From what I can see, you can use a LiPo battery (which you say isn't an option), 5V @ >2A on the USB connector, or 5V->6V @ >2A on VIN.
If you applied 12V, then I would think that the overvoltage has fairly quickly aged/destroyed part of each of your boards.
Vin has a range of 5V to 6V as per my quote from the reply #4 above (quoted directly from the Arduino MKR 1400 GSM specs on the website). If you've applied 12V to Vin, then it's very likely the cause of the damage. You might be able to replace the PRTR5V0U2X, but that may be a fruitless exercise if the rest of the electronics has also been damaged.
You could speak to Arduino Customer Services / Tech Support and see if they can advise you further but I suspect that applying 12V is the root cause.