Different robots.txt for subdomains?

Question

htaccess

There is a shop with subdomains. It is necessary that each subdomain has its own robots.txt
So that when a search engine requests robots.txt for St. Petersburg, it receives robots_spb.txt, but the path remains robots.txt
What am I doing wrong?

# Moscow
RewriteCond %{HTTP_HOST} ^www.*$ [NC]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /.*robots\.txt.*\ HTTP/ [NC]
RewriteRule ^robots\.txt /robots_msk.txt [NC,L]


# Saint-Petersburg
RewriteCond %{HTTP_HOST} ^spb.*$ [NC]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /.*robots\.txt.*\ HTTP/ [NC]
RewriteRule ^robots\.txt /robots_spb.txt [NC,L]

# Novgorod
RewriteCond %{HTTP_HOST} ^nnv.*$ [NC]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /.*robots\.txt.*\ HTTP/ [NC]
RewriteRule ^robots\.txt /robots_nnv.txt [NC,L]