Link to Advanced permissions management disappeared from SharePoint Online Manage Access menu

Yes there are other posts with this. Microsoft guy chimed in said it shouldn't be that way, so my guess is it's a bug. Targeted release tenants are getting this, so you can get it back if you go back to normal release, but you'll lose new shiny's. Kind of the trade off for targets release. If it's affecting you in a negative way I would suggest putting a request in so hopefully it gets looked at quicker if it gets reported. 

Here is the other thread.
https://techcommunity.microsoft.com/t5/SharePoint/Sharepoint-Online-Permissions-options-changed-badly/m-p/293555#M24083