diff --git a/app/core/render_engine.py b/app/core/render_engine.py
index 95d4253..e7dde06 100644
--- a/app/core/render_engine.py
+++ b/app/core/render_engine.py
@@ -595,6 +595,16 @@ class PythonRenderEngine:
             soundfont_bank = se.get("soundfont_bank") if se.get("soundfont_bank") is not None else track.get("soundfont_bank", 0)
             soundfont_program = se.get("soundfont_program") if se.get("soundfont_program") is not None else track.get("soundfont_program", 0)
             soundfont_id = se.get("soundfont_id") or track.get("soundfont_id", "")
+            instrument_program = track.get("instrument_program")
+            # GM-only track (instrumentProgram, khong synth_engine/soundfont_id):
+            # preview choi qua WASM default font -> export phai dung default SF2
+            # + program do, khong roi xuong oscillator synth (TIMBRE bug).
+            if (not soundfont_id and not instrument_id
+                    and instrument_program is not None and instrument_program != ""):
+                soundfont_id = "_default_gm"
+                soundfont_bank = 0
+                soundfont_program = int(instrument_program)
+
             is_percussion = track.get("is_percussion", False) or (soundfont_bank == 128)
             midi_channel = 9 if is_percussion else (_channel_counter % 9)
             if not is_percussion: