Backups list in Server app

I just recently did this (last night) and it is working fine for me. Perhaps if I go through the steps I did and you can compare to how you did it.

1. Have external HDD connected to Mac mini server. External HDD is called 'Data'
2. As local server administrator made folder on Data (external HDD) called 'TimeMachine' as I did not want external client Mac machines TM backups to go into root of drive
3. Went to Time Machine made a destination and choose folder created in step 2 and turned on Time Machine service on server. When this action was done server automatically setup share point with relevant permissions.
4. At client machine System Preferences | Time Machine
   1. Choose 'Select Disk...'
   2. 'TimeMachine' automatically showed up in 'Available Disks'
   3. Select it and choose 'Use Disk'
   4. For account I chose account of the user of that client machine, not the server administrator account. User chosen has account on the server and is in the group 'Staff' which has R/W access to the designated Time Machine backup sharepoint & folder.
   5. Time Machine backup automatically started
5. After client backup(s) finished I see them in the server Time Machine Backups list. Shows computer name, owner is user account which uses that Mac.

Seems to work fine.

You won't see backups for the local server in the Time Machine service backups list because the server uses Time Machine directly. That is, to run Time Machine on the server machine itself backing up to a HDD (external or internal) you don't (need to) use the Time Machine service, you just enable Time Machine on the server itself by going to System Preferences | Time Machine.

The TM service is a service — as in client/server. When you use it as a service and external machines (for example an iMac) connect to the TM service sharepoint and send Time Machine backups there then you will see them in the backups list.