Check that the result is null for accepted blocks instead of true. GBT returns {"result": null, "error": null, "id": null} if the block is accepted.

src/bitcoin.c

@@ -405,7 +405,7 @@ retry:
 		}
 		goto out;
 	}
-	if (!json_is_true(res_val)) {
+	if (!json_is_null(res_val)) {
 		res_ret = json_string_value(res_val);
 		if (res_ret && strlen(res_ret)) {
 			LOGWARNING("SUBMIT BLOCK RETURNED: %s", res_ret);