For bitset single bit, first reset, then set the bit.

src/nrnoc/eion.cpp

@@ -431,6 +431,7 @@ void nrn_check_conc_write(Prop* p_ok, Prop* pion, int i) {
     for (k = 0, j = 0; j < n_memb_func; ++j) {
         if (nrn_is_ion(j)) {
             assert(k < max_ions);
+            ion_bit_[j].reset();
             ion_bit_[j].set(k);
             ++k;
         }