I'm trying to connect power to Raspberry Pi 3B+ directly without USB cable, as in this question. I've tried both connecting the 5V and GND extension pins as in this answer and connecting to PP2 and PP5 according to this answer. But I get low power warning even though the voltage should be enough, I've even increased up to more than 6V (I've measured 5.9V when using the USB adapter). I've tried both a Mean Well LRS-50-5 switching power supply (5V, 10A adjustable ±0.5V), and one from Adafruit (5V, 10A). I'm attaching cables on a breadboard, but I've measured the voltage directly on the Pi as well. What could be the problem?