This was first reported in this Forum post:
To replicate:
  1. Create a new WordPress Subdomain Multisite within Local
  2. Log into the WP admin for the site and create at least one subsite
  3. Within Local, click the "Sync Multi-site Domains to Hosts file"
  4. Click the "Trust" button for the site's SSL certificate
  5. Navigate to the subsite's url over HTTPS and examine the certificate.
Note that the certificate does not cover the subsite because there is not wild-card certificate. This is a regression since previous versions of Local did create this wildcard certificate (see attached screenshots taken from the forum thread)