When using the portal to create reserved IPs the name gets prepended with "Group <groupname> <reserved ip name>". Then, when it's displayed it only shows what was entered, so it's very deceiving. If you use powershell to view your list of reserved IPs you can see the actual name and it's usable.
This was an issue with classic IPs and how they appear in ARM. We recommend using ARM for future deployments.
- Anavi N [MSFT]