@dawn sun Thank you for your help. So I assumed in this chat, the Custom Instruction I used is triggering.
here is what I tried: start a new conversation with no CI, add contents from the previous triggering CI. chatgpt will ignore the content partially (when i ask GPT what is the last few words it sees from my prompt, it will be somewhere in the middle, cut off mid-sentence). I iteratively removed paragraphs from the triggering CI (which is now in the 1st prompt), and can reach a point when chatGPT will tell me that it can see my prompt completely (when the last words it sees is indeed the last few words of my prompt).
I think the problem here is not my contents being flagged out for violating policy (i am writing sensitive stuff related to fictional protagonist with trauma history and struggling with mental issues; it's just my personal way of coping), but the fact that it truncates stuff instead outright flag out as policy-violating or refuse to answer me. the unpredictability of its behaviour is the real issue.