zlib, pigz, header issue

I have a problem compressing data with zlib level 8.

- 'pigz' creates file with '78 5E' header.
- 'zlib-flate' (qpdf) creates almost identical file except it has '78 DA' header (same as level 9).
- .psarc seems to use '78 DA' header too.

so, there seems to be a bug in "pigz v2.4" that makes wrong header?

(using these options:
cat file.dat | pigz -8 --zlib --no-time --no-name >file.zlib
)

thanks.