Ghostrider ignores affinity

How can I make sure, that Xmrig doesn't use specific thread numbers when mining Ghostrider?

This works perfect for all other algorithms by editing the config.json but for Ghostrider. 
It seems, that it's not possible to define a proper affinity (like affinity 0xFFF5, 14 threads, on an 8core/16thread AMD for example). The usage is not predictable - at times it uses only half of all threads, at times all threads.
 