Hi there, searched around and found some similar topics but nothing exact.
maindomain . com pointing to shopify
blog.maindomain . com pointing to bluehost
www is an alias of shops.myshopify.com
SSL works on the maindomain . com pointing to shopify, although I think all shopify shops have SSL thru shopify.
SSL is not working on blog.maindomain . com that is pointing to bluehost.
I saw that some people had issues if it was going to many subdomains deep as in www.blog.maindomain . com but mine is showing as blog.maindomain . com so shouldn’t be an issue.