I had to help a colleague recently export group members of a group of 6k people, it turns out that there is a limit of 5000 member objects for a group. I did some searching online and I found a solution, however I have lost the original link (I will update this when I find it) which showed the below method.
Get-ADGroup "PV-BlueCoteInternetAccessBasic" -Properties Member | Select-Object -ExpandProperty Member |Get-ADUser
This worked a treat. If you want them the details of the limit are here