When using Set-ACL to modify the ACL on the root folder of a share, it breaks permissions inheritance so you can eventually be locked out of the folder. When I tried it on my 2012 lab, the owner could not be displayed, and every inherited permission was removed (after I took ownership back agin).
By modifying the ACL, in any way(edit a non-inherited permission, disable and re-enable inheritance etc) resets the ACL back to the default.
This does not happend when using Set-ACL on a folder inside a share (ex. \\server\mysharename\thisfolder )