It was reported in https://www.mail-archive.com/haproxy@formilux.org/msg33009.html (Re: Small question regarding the sub-dir match (i.e. -m dir) with regard to "rooted" patterns) that the documentation regarding the sub-dir match is a bit unclear in that it could be misunderstood to only match directories at the prefix:

On Mon, Mar 11, 2019 at 12:58 PM Tim Düsterhus <snip> wrote:

The documentation only talks about a slash-delimited value, not about
being at the beginning.

Technically yes, it works as documented, except the documentation is
misleading by using the work subdir match right in the beginning...

Moreover the name of the operator is dir which (my guess is that)
most would just mistakenly associate with a file-system, where they
would expect to match a prefix.

I would strongly suggest adding an explicit warning in the
documentation about this pitfall.

Consider amending the documentation.