Odd. It works directly in CSOM - see below. The issue seems to be with Get-PnPWeb not returning the context. I haven't got the latest version set up on my PC to test but can't see any obvious issue. I also noticed that you can get the count. I'd use CSOM fully for now and raise an issue with details on https://github.com/SharePoint/PnP-PowerShell/issues.
Id Title LoginName -- ----- --------- 3 Excel Services Viewers Excel Services Viewers 7 McGraw Hill Members McGraw Hill Members 5 McGraw Hill Owners McGraw Hill Owners 6 McGraw Hill Visitors McGraw Hill Visitors