Tags auto fetched by "git fetch origin" but not "git fetch origin main"

In using `git fetch [<options>] [<repository> [<refspec>…]]`,
- when a branch is specified as <refspec>, no tags are fetched automatically;
- when no <refspec>s are specified, tags are fetched automatically.

This reproduces with both glob (`+refs/heads/*:refs/remotes/origin/*`)
or restricted (`+refs/heads/main:refs/remotes/origin/main`)
`remote.<name>.fetch`.

From the git-fetch doc in Git v2.46.0 [1], I don't understand why they
behave differently.

DESCRIPTION
[...]
By default, any tag that points into the histories being fetched is
also fetched; the effect is to fetch tags that
point at branches that you are interested in.

<refspec>
[...] Since
Git version 2.20, fetching to update `refs/tags/*` works the same way
as when pushing. I.e. any updates will be rejected without `+` in the
refspec (or `--force`).

CONFIGURED REMOTE-TRACKING BRANCHES
[...]
This configuration is used in two ways:

* When `git fetch` is run without specifying what branches
and/or tags to fetch on the command line, e.g. `git fetch origin`
or `git fetch`, `remote.<repository>.fetch` values are used as
the refspecs ...

* When `git fetch` is run with explicit branches and/or tags
to fetch on the command line, e.g. `git fetch origin master`, the
<refspec>s given on the command line determine what are to be
fetched (...). The `remote.<repository>.fetch` values determine
which remote-tracking branch, if any, is updated.

Steps to reproduce

# prepare a local upstream and fork it
git init local-upstream && cd local-upstream
for i in 1 2 3; do git commit --allow-empty -m "commit $i"; done
git tag v0.3
cd ..
git clone file://$(pwd)/local-upstream local-fork
# now local-fork has all three commits and a tag v0.3

# add more commits and a new tag to upstream
cd local-upstream
for i in 4 5; do git commit --allow-empty -m "commit $i"; done
git tag v0.5
cd ..

# try to fetch new commits and the new tag v0.5
cd local-fork
# this fetches commits only
git fetch origin main
# this finally fetches the tag new v0.5, but why?
git fetch origin
cd ..

[1] https://git-scm.com/docs/git-fetch/2.46.0

Yukai