I can reproduce this, seems like an issue or no supported now. But I was able to do this in another way.
Create a PowerShell endpoint using PowerShell core
Then connect to that end point from 5.1
# from remote server
New-PSSessionConfigurationFile -Path C:\Temp\ps6.pssc
Register-PSSessionConfiguration -Path C:\Temp\ps6.pssc -Name ps6
# then from client
Enter-PSSession -ComputerName server -ConfigurationName ps6