Back in 2022 libtorrent implemented memory mapped IO as part of v2 [1]. Unfortunately, it didn't go so well. Memory usage went through the sky, leading to performance degradation and crashing [2]. The issue is still open in the project to this day, and many programs have stuck with the v1.2 library instead.
It looks like they are headed to a multi-threaded pread implementation now [3] and someone has created a patch to tweak the current mmap fallback that uses pread to perform better in the meantime [4].