could it be a plugin or something causing this? is this normal?

I have created a new category for "example-to", then I added a new post and added it to this category, the URL structure didn't change, then I added site.com/example-to/cluster and the URL changed like before / to -

yes; i'm using wordpress

Question. I have created a pillar page "ex.com/example/" then I created a cluster post but when I enter URL in the permalink section "ex.com/example/cluster1" and click ok, the url changes to "ex.com/example-cluster1". Why is that? Would a pillar page still work by having the URL structured this way? It doesn't look good though.