Dear Zhiyu Xu,
Indeed, this is a real issue. Thanks.
It turns out that the 186W(d,2n) channel is currently heavily underestimated by FLUKA. This can be explained by a slightly excessive Coulomb barrier in the model, preventing the stripped proton from penetrating the target nucleus.
This issue mainly concerns very low-energy deuterons on high-Z target nuclei (though not necessarily all reaction channels).
A solution is being worked on.
Cesc