[Qwen3.5][MTP] Preserve online NVFP4 draft quantization for mixed checkpoints (#35545)

python/sglang/srt/configs/model_config.py

@@ -1516,14 +1516,14 @@ class ModelConfig:
                 "quant_method", "" if not self.quantization else self.quantization
             ).lower()
 
-            # ModelOpt FP4 checkpoints quantize only the target model; an
-            # embedded MTP draft may stay unquantized, so an explicit
+            # ModelOpt FP4 and mixed checkpoints can quantize only the target
+            # model; an embedded MTP draft may stay unquantized, so an explicit
             # nvfp4_online opt-in for the draft wins over checkpoint detection.
             # The online loader rejects already-packed weights at load time.
             preserve_online_draft_quantization = (
                 self.is_draft_model
                 and self.quantization == "nvfp4_online"
-                and quant_method == "modelopt_fp4"
+                and quant_method in ("modelopt_fp4", "modelopt_mixed")
             )
             # An explicit online-requantization request (e.g. quark_mxfp4 on top
             # of an NVFP4/mixed checkpoint) must not be overridden back to the