Likely because SPP bakes in "detail recovery" with a high pass filter in its early processing stage alongside noise suppression. SPP does a lot beyond prepping and calibrating the raw layer data. Some aspects of that can be desirable but for portraits I find the lack of control over that processing step to be a real downer.
I did not start this project but here are some changes I cooked up, some are seeking parity and some are net-new. https://github.com/sagwaco/x3fuse-core/pull/23 https://github.com/hank-bond/x3fuse-core/pull/7
I am currently working on recreating more aspects of classic X3F processing and it's looking like the future of Foveon is very bright.