I’ve been having this issue for a couple days now. I’ve been working on a project for 2+ years across multiple conversations. I’m at the point where I need to reference data from across numerous conversations, but chatGPT can’t reliably look at canvases across multiple conversations. We HAD developed a workaround (copying those canvases and pasting them into “data collator” canvases in a new conversation) and it was working great . . . until yesterday. It took me through a bunch of steps (checking word count, etc.) to determine that there was nothing wrong on my end.
It also suggested coming here, but chatGPT was of the opinion that OpenAI doesn’t have an active public information source for canvas issues. 
It did offer helpful suggestions on how I could use other apps (MS WORD) to do all the work myself the old school way. 