Custom Instructions disappeared

Last week, I discovered the "Custom Instructions" personalization option in M365 Copilot chat and, after some trial and error, got things dialed in such a way that the chat experience got 10x better for my use cases. 

Today, I noticed that the chat responses had reverted back to my pre-customization experience and when I went to verify my prompt was still being applied I found that the setting was no longer there... where I used to be about to got to Settings -> Custom Instructions, the only option under Settings now is "Manage Copilot agents with external data access."

Frankly, now that I've had a taste of it, the "normal" Copilot experience is just frustrating me... like to the point where I'm about to start a ChatGPT trial to see if we can just move over to that tool before our Copilot renew comes up.

How can I bring that "Custom Instructions" option back?