WordPress install in subdirectory

So I decided to try to move yet another WordPress website to our main server, and since it has never been a multisite version, I decided to put it inside a subdirectory and create a .htaccess redirection. So

– a DNS alias is set up to the server root (where our main wordpress multisite lives)
– the website is installed inside a subdirectory, call it subdir
– the root htaccess checks for domain name and redirects:
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.xy
RewriteCond %{REQUEST_URI} !^/subdir/
RewriteRule ^(.*)$ /subdir/$1 [L]
– the subdir htaccess is a common WordPress htaccess rules file

Everything runs smoothly except for media files inside /subdir/wp-content/uploads/.* – these files are getting 404. I found out that all such links were in fact treated as pages. Furthermore, debugging both htaccess files with minimal example approach has shown that
– links that begin with wp-content/uploads get trimmed, so that domain.xy/wp-content/uploads/dir/file.png is ALWAYS dir/file.png in BOTH .htaccess files (redirected all to a custom php script using QSA,L that printed out $_GET, even as the first rule of the parent htaccess).
– other links, for example domain.xy/subdir/wp-content[...]file.png work flawlessly
– with RewriteRule (.*\.(js|css|ico|gif|png|jpg|jpeg|webp)) /subdir/wp-content/uploads/$1 [L] all seems fine but this is not a fix since media files can be elsewhere
– there are no .htaccess files in the child directories
– there are no problems with file privileges

I have no access to the apache/php configuration.

Simply, wp-content/uploads part vanishes from REQUEST_URI for the domain – if it starts with it. Thank you for any help.