Originally compiled my own libRmath-nim.so but tried using the nimble-pkg provided copy instead (via LD_LIBRARY_PATH). Still same result. My C compilers are slightly older. Clang 3.6, GCC 4.8.4. I'd be surprised if that made a difference. I'm assuming, @andrea, that gibbs succeeds for you with -d:release (while sir fails): as per @sdwfrost`s experience?
Even if I can't replicate the issue, there are a few other things I can try.