Avoid trailingslash redirect

[Matt .]

when I have a rewrite rule which does:

subdomain.domain.tld

with a destination host

domain.tld/subdomain and I place a $1 like

domain.tld/subdomain$1

my subdomain.domain.tld gets redirected to domain.tld/subdomain/

so there is no rewrite anymore but all my links behind
subdomain.domain.tld/foo work perfectly.

When I place a slash in front of the /$1 my /foo doesn't work anymore.

When I place nothing at all I can visit my subdomain.domain.tld and
get the page but no links work anymore as I'm missing the $1

I am using this vmod:
https://www.varnish-cache.org/vmod/database-driven-rewrites

Anyone knows why this happens ?

Cheers,

Matt