Yes, this is caused by mod_dir appending a slash (with a 301 redirect) to the directory after the rewrite has occurred, exposing the internally rewritten URL/directory.
The canonical URL therefore needs to be /articles/ (with a trailing slash), not /articles. We can correct this with an external redirect before the rewrite occurs.
(This avoids you having to disable the DirectorySlash - which would still leave you with a canonicalization / duplicate content issue.)
For example, before the existing rewrite, test to see if the requested URL-path (that is missing a trailing slash) exists as a directory in the /sub directory and append a slash if that is the case.
# Redirect to append trailing slash if exists as a dir inside "/sub"
RewriteCond %{HTTP_HOST} ^sub\.mydomain\.com [NC]
RewriteCond %{DOCUMENT_ROOT}/sub/$1 -d
RewriteRule ^((?!sub/).*[^/])$ /$1/ [R=301,L]
As an additional optimisation, you can avoid unnecessarily performing a filesystem check (which are relatively expensive) on static assets (that naturally do not end in a trailing slash) by excluding URLs that look like they have a file-extension. (This assumes you don't have physical directories that have, what looks like, a file extension, eg. /sub/somedir.xyz)
Add the following as the 2nd condition (before the filesystem check) in the above rule:
RewriteCond %{REQUEST_URI} !\.\w{2,4}$
Aside:
RewriteCond %{HTTP_HOST} ^sub.mydomain.com$ [NC]
RewriteRule ^((?!sub).*)$ /sub/$1 [NC]
You should probably be using the L flag on this RewriteRule directive. (And the NC flag should be unnecessary.)
The regex ^((?!sub).*)$ excludes any URL-path that simply starts sub, which would include /subfoo and /subbar, etc. (which naturally prevents these directories from being accessible in the /sub directory). Any valid request would start /sub/ (with a trailing slash), so should be included in the negative lookahead, as I did in the rule above.
If not already, consider also redirecting to remove /sub/ from direct requests if this directory should be exposed/discovered.
