If your custom Thank You page does not display when you test it, it’s likely that another layout is interferring. Any other layouts using All Pages or Checkout in the Conditions could potentially do this.
You could go in and exclude the WooCommerce Thank You page from any other layouts using All Pages or checkout, but the easiest way to deal with this is just to move the Thank You Page Layout to the end of the layouts. Apart from the global layout, layouts further down the list take precedence over ones higher up.
So just make sure the Thank You Page layout is further down than any other layouts using All Pages or Checkout in the Conditions, and Save the Layout Order, and there should be no conflict.