(I have the 13" AMD but anyway) The default fan control logic is based on a temp sensor on the motherboard near the CPU, but not the CPU temp itself. So if you have just one core pretty active, and the rest mostly idle, overall power usage won’t be very high, so overall heating of the rest of the laptop won’t be much, so the fan will stay off, while the CPU will get up to around 100°C then thermal throttle. So this is all by design, supposing most people want the fan to just stay off if possible (if the laptop won’t become uncomfortably hot for you).
But you may prefer for the fan to kick on when the CPU gets hot, even if the overall laptop isn’t hot yet, and that is possible - using ectool. There are a number of threads about this, I’m not sure if I can find the original, but here’s one of the more recent ones where I posted the values which worked for me: AMD FW13 fan behaviour and ramp-up times - #13 by pierce (and about ectool see: Exploring the Embedded Controller)