[BUG?] remote prune origin interacts badly with clone --mirror and multiple remotes

[git version: next as of yesterday afternoon]

If I clone a repo with git clone --mirror, and add other remotes later, 'git remote prune origin' deletes all branches and tags of the other remotes.

Easily repeatable example:

[core]
	repositoryformatversion = 0
	filemode = true
	bare = true
	logallrefupdates = false
[remote "origin"]
	url = git://github.com/git/git.git
	fetch = +refs/*:refs/*
	mirror = true
[remote "peff"]
	url = git://github.com/peff/git.git
	fetch = +refs/heads/*:refs/remotes/peff/*

'git remote prune origin' will delete all peff's branches in this case. I'm guessing the wildcards refs/* and refs/remotes/peff/* interact badly in some place, and I'm trying to understand builtin/remote.c to see if I can fix it, but haven't gotten very far yet.

git fetch --prune origin and git remote update --prune also show this behaviour.

git remote prune peff does not delete non-peff branches in this scenario, further strengthening my belief that the refs/* and refs/remotes/peff/* wildcards interact badly with prune.

Or is this considered normal behaviour and is what I'm trying to do simply unsupported? In that case a warning would be welcome when adding remotes to a --mirror'ed repository.