I ran into an issue yesterday that had me completely stumped and I had to pull in cPanel support for a second pair of eyes (and of course they fixed the issue in record speed). A customer had asked me to help them convert standalone WordPress to a Multisite. I had done everything properly with converting the install (guide at https://codex.wordpress.org/Create_A_Network if you ever need to do that) all the way down to setting up wp-config.php, .htaccess, etc. The other piece is for wildcard subdomains you have to create a subdomain like this in cPanel:
Done and done. But after create a site it was not resolving, nor were any of the test sites we created with it. In fact even creating another generic subdomain didn't resolve. It was like DNS was just broken. I checked all the usual suspects, they were using our nameservers, intodns.com gave a clean bill of health, but a dig command showed no A record for *.domain.com even though cPanel clearly had one.
The culprit ended up being an additional record that for some reason cPanel created: www.*
That is not a valid A record and was essentially breaking the DNS zone so no new records would load. All I had to do was delete that one zone record while leaving everything else as it was and the subdomains started working.
Figured when Timmmmy gets stumped for hours and has to call in the big dogs it warrants a blog post :)