`mrb_str_pool` can embed one more byte; ref
MODIFY
src/state.c
src/state.c
@@ -170,7 +170,7 @@ mrb_str_pool(mrb_state *mrb, mrb_value str)
len = s->as.heap.len;
}
- if (len < RSTRING_EMBED_LEN_MAX) {
+ if (RSTR_EMBEDDABLE_P(len)) {
RSTR_SET_EMBED_FLAG(ns);
RSTR_SET_EMBED_LEN(ns, len);
if (ptr) {

`mrb_str_pool` can embed one more byte; ref #4626