I ran into this issue when I was editing a AAD Connect syncronization rule. If you edit a AAD syncronization rule and set the same precedence as an already existing syncronization rule you will get a SQL deadlock warning.

A deadlock occurred in SQL Server while trying to acquire an application lock.,Microsoft.IdentityManagement.PowerShell.Cmdlet.AddADSyncRuleCmdlet

When I did it I did a copy of the original rule and then disable the original and set the new duplicate to the same precedence as the original, that was a bad idea. So check with another precedence. And let this be a lesson.