Hi,
I've seen this come up with a few people recently, so you're not alone.
What's likely happening is that Copilot doesn't keep the whole conversation in view. As a chat gets longer, or covers a few different topics, it works from a condensed version of what's been said, and sometimes the more recent topic is the one that gets dropped. That's why it falls back to Topic A, and why it can tell you Topic B never happened. As far as Copilot is concerned, it's no longer there. Pasting it back in fixes things because you're putting it back in front of it.
The timing you mention fits with some recent changes too. A major Copilot update began rolling out on August 18, 2026, and Microsoft did say there could be some gaps while things transition. I can't say for certain that's the cause, but it lines up.
The biggest thing that's helped for me is starting a new chat whenever I switch topics. It feels like extra effort at first, but Copilot is much more reliable when each chat sticks to one subject. If you do want to stay in the same chat, it helps to remind it what you're talking about, something like "going back to the budget report we just discussed". I'd also avoid switching between Work and Web, or jumping between agents, halfway through a conversation, and it's worth trying the same thing in a different app (Teams versus the Copilot web app, for example) to see if it behaves any differently.
If it keeps happening, give the dodgy responses a thumbs down with a quick note, as that goes straight back to Microsoft. If you're using a work account, your IT admin can also check Service Health for any known issues and raise a ticket if needed.
Hope that helps, and let us know how you get on.