Model: Raspberry Pi 3b+ Raspberry pi 4.19.66-v7+ Raspbian Buster July 2019 edition Offical PSU. I have an Arduino uno powering off one of the USB ports on the PI. Nothing else attached.
I have my raspberry pi plugged into the official USB PSU which plugs into the mains power through a powerboard or through the wall socket. If I plug anything into one of the other power sockets or any of the free sockets on the powerboard the raspberry pi reboots. I thought it was a faulty PSU so I tested another official PSU. It still has the same problem. I've tried other sockets and powerboards around the house. Same problem. If I have any of the other devices plugged in and then turn the raspberry pi on it continues to function as normal. It only reboots when I plug one of these devices in after the raspberry pi is on. I've tried a number of different devices, all-cause it to reboot.