Interesting. I've just tried a Onewire scanner on a NANO. It detects two DS18B20 when they are on pins 6, 12 or 19. But they aren't detected when they are on pins 20 (A6) or 21 (A7). There must be something different about those pins. I tried a 2k2 pullup instead of 4k7 and it didn't help.
Pete