Subdomain redirecting to domain - wordpress

Question

Currently I am setting up a WordPress Multisite. Every customer will have their own subdomain.
So in this example our site is called: www.example-site.de. One customer will have his own domain called "www.customerxy.example-site.de"
Unfortunately, sometimes the subdomains redirect to the main domain. If I type "https://customerxy.example-site.de" I was not able to reproduce the problem. Just when I typed "customerxy.example-site.de". I do not know if this has anything to do with it, but this was something I noticed.
I already wrote that it could be, because subdomain and main domain are pointing on the same folder. The .htaccess file shows the following:
"
RewriteEngine On
RewriteCond %{SERVER_PORT} !=443
RewriteRule ^(.*)$ https://customerxy.example-site.de/$1 [R=301,L]
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
# END WordPress
"
If some1 has any idea how to fix this that sometimes the subdomains redirect to the main domain, I'd be so glad. Thank you for every help in advance!
Best regards
Benjamin