It seems that meson's 'install_man()' forces compressing manpages to .gz and has no way to disable this behavior. Looking at mesonbuild/backend/backends.py, the generate_man_install() method forcedly appends '.gz' to all destination filenames, enabling the implicit compression in install target. Please fix it not to compress manpages, or at least to make it possible to disable it.
Oh, it seems that hanetzer already reported it upstream.
The compression has been removed in master, so hopefully bump to the next release will fix it.
This is fixed in 0.49.0.