Welcome to Flyer Talk pelehton

I think I'm seeing the same availability oddity.

Have an itinerary ready to book. Check availability and tool says all okay except one flight not available. Change that one flight and try again. Then tool says all okay except one flight not available, and that one flight is a different flight (not connecting to or from the just changed flight). Change that flight and try again to book. Now the first flight which I'd changed is no longer available. Change back to my original selection and try again. That flight is now okay but the second changed flight is no longer okay.

Given the flights are unrelated to each other it cannot be a case of married segment logic.

In my case the problem flights were LHR-JNB and SYD-AKL.