diff --git a/cpp_engine/tests/bench_qwen_gqa_prefill.cpp b/cpp_engine/tests/bench_qwen_gqa_prefill.cpp
index 3133f1b5..f5dd247d 100644
--- a/cpp_engine/tests/bench_qwen_gqa_prefill.cpp
+++ b/cpp_engine/tests/bench_qwen_gqa_prefill.cpp
@@ -55,11 +55,11 @@ double time_case(int rows, int position_offset, int max_context, int iters,
     // baseline explicit so this benchmark cannot compare hpg6 against itself.
     setenv("DSV4_QWEN_GQA_LONG_TILE", mode == Mode::Base ? "0" : "1", 1);
     unsetenv("DSV4_QWEN_GQA_FLASH_TILE");
-    unsetenv("DSV4_QWEN_GQA_MMA_TILE");
     if (mode == Mode::Flash) setenv("DSV4_QWEN_GQA_FLASH_TILE", "1", 1);
-    // The MMA dispatch is checked before flash and long, so this one variable
-    // selects it regardless of the others.
-    if (mode == Mode::Mma) setenv("DSV4_QWEN_GQA_MMA_TILE", "1", 1);
+    // MMA is the production default and is dispatched before flash and long, so
+    // unsetting its variable would silently run MMA for every mode and make all
+    // four rows identical. It has to be switched off explicitly instead.
+    setenv("DSV4_QWEN_GQA_MMA_TILE", mode == Mode::Mma ? "1" : "0", 1);
     std::vector<uint16_t> host_q(q_elements);
     std::vector<uint16_t> host_kv(kv_elements, 0);
     for (uint16_t& value : host_q) {