Qwen3.5 models spit gibberish on long prompts

Hot take: this smells like a runtime/config bug first and a model-quality problem second. Qwen3.5's own docs advertise a 262K-token default context and recommend keeping at least 128K for thinking, so collapsing around 50K is well below the published envelope.

• –Local stacks like LM Studio, GGUF builds, and llama.cpp derivatives can diverge from upstream serving recipes on chat templates, reasoning parsers, and context management.
• –The fact that the gibberish starts in the thinking trace points toward cache/state corruption or context-window handling, not just a bad sampling preset.
• –GPT-OSS-120B handling the same prompt cleanly suggests the input itself is not inherently pathological.
• –If others can reproduce this on official Qwen presets, it deserves a backend bug report with exact quantization, context length, and template settings.