Hi Kevin,
I just spoke to a member of the technical team and they advised that this is currently a limitation of the Quickpay form. If there are more than 10 tabs open for bills or invoices, you can then only perform quick pay on the most recently opened one. It's something that may be addressed in future, but it probably won't get looked at until after the current improvements are finished.
I'm sorry I couldn't be more helpful. If you find there are any other issues with this, please do let us know.
All the best,
Natasha