Here is a little report on my progression. In short : it still doesn't work!
I checked that the pin "works" correctly by connecting a relay to test in OUTPUT mode, and a push button (to test in INPUT) : it works!
I checked, double checked and triple checked the setup (wires, connections,...), everything is good.
I ran the same code on a Wemos D1 Mini (ESP8266) : it works.
At this point, I'm lost? The pin and the code seem good, but it can't detect the sensor.
One thing I notice is that this weird whistling sound coming out of the board (not sure if it comes from the speaker or not) when the OneWire lib bitbangs the OneWire protocol on the pin.
So, I made another test program that toggles the pin rapidely (at the speed of the onewire lib - ~50µs) : I hear the sound!
I'll try to find an oscilloscope to check the signal on the pin, as I suspect that "something" is going wrong when the pin is driven at "high" speed.
In the meantime, I hope that someone with more electronical knowledge will help me find a solution!