Today one of my colleague reached out to me for an help to drop a subscriber from an publisher which is participating in Always On. We seen same behavior in UI and in T-SQL statement. As a start I was looking online for help to resolve this issue. I found an KB article from Microsoft which describes this is an known issue and fixed in the below versions of SQL Server.
*) SQL Server 2016 SP2 CU1 and above
*) SQL Server 2017 RTM CU7 and above