SQL Server Agent Invokes Failed 'sa' Login
After a few days of research, it looks like this issue is caused by a bug in SQL Server 2016. In the following thread that describes this issue, other DBAs are experiencing the same thing. One person troubleshooted the issue with a Microsoft support engineer who was confident it was a bug.  Maintenance plan - Login failed for user 'sa' on master database

BUG: Use SQL authentication when creating a maintenance plan through the maintenance plan wizard. Run the maintenance plan. It will succeed, but after it succeeds, it will log a number of failed logins.

WORKAROUND: Use Windows authentication instead of SQL authentication. If you have already created the maintenance plan, you will need to right click on it and select modify. Look for "manage connections" at the top window. You will be able to modify the local connection here. You will need to be logged on your machine with that domain account and that domain account will need the proper SQL privileges.