it was probably there for a while

Nah, it might not have been. Have been dealing with federation issues that I think may only have sorted themselves out mere minutes before your comment.

I’m sure there’s lots of stuff about them spying with minimal searching

Eh, not good enough. They’ve got a lot of spying, but this is a specific claim. It needs to be supported by specific evidence.

It’s like when people claim that Google devices are constantly recording audio and sending it back to base. Nobody has ever found evidence of it, and claims that they are are usually better explained by things like “they are recording your geolocation, and the geolocation of people you’re with, and the things that the people you’re with are Googling”. That’s enough data on its own, there’s no need to reach for conspiratorial claims that lack evidence.

If there’s actual evidence that the browser itself is sending tracking data back while in Incognito which links your non-incognito profile to what you’re doing incognito, I’m concerned. But if it’s just assumptions people are leaping to, I’m gonna go back to Occam’s razor and make fewer assumptions.