I think that is because each rename causes the zip to be updated and in turn the folder to be re-read, which cancels the inline rename.
We've been thinking about a batch rename mode where the work is postponed until you rename the last file, then all done at once. That would also improve performance when doing wildcard renaming inside archives. But we haven't got to it yet, I guess partly because it's fairly rare to rename lots of stuff within an archive. It'd be good, though.
Renaming in zip is a fairly expensive operation (because basically the file is extracted, renamed, the original deleted and the renamed file re-added), so I think it takes long enough that the progress bar appears, which steals the focus from the rename field and causes it to exit.