Steps to reproduce:
Create a cluster named "my-cluster"
Add an app repository
It is associated with the cluster, yeah!
Try to deploy an app from the catalog and you have "my-cluster" as an option
Delete the cluster (not the repository)
Try to deploy an app from the catalog and there are no cluster options (because there are no clusters available)
Create a new cluster named "my-cluster"
List app repositories - it says that the repository is assigned with 1 cluster
Try to deploy an app from the catalog and there are still no cluster options :(, even though the list claimed that the repository was assigned to a cluster
List the repositories and edit it
Don't change a thing, just click "Save"
Now the repository is actually assigned to the cluster and can actually deploy
Workarounds make me sad.