Bzip2 is block-based compression with large (up to 800 Kb) size of block. It means that while decompressor not read enough data it can't produce any output (actually this issue exists for other compression algorithms too, but less frequent). And the read1() method which makes only one low-level read can return empty bytes when there is still data available. This issue was fixed for gzip, bz2, and lzma modules in issue15546, but left in the zipfile module. In zipfile this bug less catastrophic because other read functions do not use read1() directly, but use lower level internal function.