Powershell Set-Acl use without changing owner?

I have a folder, and I am trying to grant full permissions to another user, like so:

$Acl = Get-Acl "C:\MyFolder"

$AccessRule = New-Object  system.security.accesscontrol.filesystemaccessrule("bobby","FullControl", "Allow")
$Acl.AddAccessRule($AccessRule)
Set-Acl -AclObject $Acl "C:\MyFolder"

The owner of the folder is "Joe".
The powershell script runs under the "SyncProcess" account.

If Joe is the owner of the folder, the script fails with the error message:
Set-Acl : The security identifier is not allowed to be the owner of this object

If I change the owner of the folder manually to SyncProcess, then the script runs fine without error.

My question is, why does Set-Acl try to change the owner?  I am not trying to change the owner, nor do I want to.  I read the script to mean, get the current Acl, add the full control permission access rule for Bobby, and then update the folder.  I don't think I am doing anything here to change the owner?

if I add $Acl | fl both before and after the AddAccessRule method call, the Owner is the same in both, unchanged.  Whe does Set-Acl think the owner is changing?

Thanks.