There is an updated pull request by zdtcd against master on the void-packages repository

https://github.com/zdtcd/void-packages man-db
https://github.com/void-linux/void-packages/pull/15737

[NO MERGE] man-db: add system account _man:_man
man-db complains that:
> mandb: the setuid man user "man" does not exist

A patch file from https://github.com/void-linux/void-packages/pull/15737.patch is attached