Thanks for the detailed explanation! I understand that additional tools and a growing conversation history can introduce complexity, making strict adherence to all instructions less reliable. The point about potential scripting abuse is also fair—predictability in outputs can be exploited in unintended ways.
That said, cross-model behavior regarding Custom Instructions should still be as predictable as possible to the user. If custom instructions are inconsistently followed based on which model is used, which features are enabled or how long the conversation has been, it makes it harder to rely on them, especially in structured workflows. While using the API provides more control, users will expect full consistency within ChatGPT itself, especially since Customize ChatGPT is exposed to them in the UI as a basic feature.