Different use patterns will result in different temperatures. Very tight math loops (no memory/IO wait) will lead to higher temperatures than something that that relies on L2/3 cache or main memory, even though they’ll both report “100% CPU use” and probably use similar amounts of power. And, different operations will produce heat in different areas of the die; depending on physical layout, some operations might generate heat in a tiny cluster, whereas some others might generate heat in larger spread out areas. Even though both of those cases might use the same amount of power and generate the same amount of heat, the temperatures will be drastically different due to the heat concentration.

Iirc the FFT step uses AVX, and on Zen 5 that’ll be AVX-512. It should keep 100% of the required data in L1 caches, so you’re keeping the AVX units busy literally 100% of the time if things are working right. The rest of the core will be cold/inactive, so if you’re dumping an entire core’s worth of power into a teeny tiny ALU, which is gonna result in high temps. Most (all?) processors downclock under heavy AVX load, sometimes by as much a 1GHz (compared to max boost), because a) the crazy high temperatures results in more instability at higher frequencies, and b) if the clocks were kept high, temperatures would get even higher.

Try LINPACK, it's even more stressful than Prime95.