Persistent Severity 016 alerts: "Change Tracking autocleanup is blocked on side table"

It seems to be a known issue and has already been recognized by Microsoft. You may read resolution as below:

Resolution

A fix for this issue is included in the following updates for SQL Server:

Cumulative Update 10 for SQL Server 2017

Cumulative Update 3 for SQL Server 2016 Service Pack 2

Cumulative Update 1 for SQL Server 2014 Service Pack 3

With this fix, you will not see continuous messages any longer. The frequency of the error 22123 messages, if any, will not be more than once every 30 minutes. To completely eliminate these messages, enable trace flag 8293.

Listed are the version which has this issues:

• SQL Server 2017 Developer
• SQL Server 2017 Enterprise
• SQL Server 2017 Enterprise Core
• SQL Server 2017 Standard Windows
• SQL Server 2016 Developer
• SQL Server 2016 Enterprise
• SQL Server 2016 Enterprise Core
• SQL Server 2016 Standard
• SQL Server 2014 Developer
• SQL Server 2014 Enterprise
• SQL Server 2014 Enterprise Core
• SQL Server 2014 Standard

Please read more from link.

Hope above helps.