Agent Pools > Assign Project button disappeared

Recently we have been increasing the number of agent pools in our Team City configuration.  Some time in the last two weeks we noticed the button to "Assign Project" on the build agent pool screen has disappeared for some pools.  Is there some condition in the Team City logic which disables this button for a pool?  For example, too many projects assigned, or too many pools assigned to the same project?

See the below screenshots for examples:

It may be worth noting some of these agent pools are based on Cloud Profiles and other pools are based on virtual machines.  However, there doesn't seem to be a correlation between the type of infrastructure setup and the existence of the "Assign Project" button.

Please let me know if this is an issue with my setup or if it's a Team City bug.