It's not a bug, they are doing it intentionally and it's awful. Here is the email I got from App Store
"Being unable to now edit screenshots while an app is in "Waiting for Review" status, is expected behavior.
Developer-facing documentation currently lists "Waiting for Review" as a state where locked metadata can be edited. Screenshots should be the only locked metadata that cannot be edited while an app is in the "Waiting for Review" status.
We are currently updating the developer resources and documentation to clarify".
This is hugely damaging to small developers. I just finished submitting a new app last night, and expected to spend all week working on screen shots and marketing text. Now I am going to have to spend the next 3-4 days getting good screenshots together just so I can resubmit and further delay my app another 3-4 days. And if we come up with better concepts for the screen shots after that, we'll have to pull it from review again, or wait through another submission cycle.
All meta data should be editable until review starts, there is no reason not to allow this and it's hugely important when you are a small shop limited resources. I don't have extra people to work on screen shots while we are doing testing & development at the same time, and I can't afford to hire someone to do it.