I am having some issues pinpointing where this issue is on the default template not showing a custom layout.
I have created a collection and modified the layout adding a custom HTML_block.
The html_block is active and saved under the default template. I can see the collection listed under the layout and Blocks manager.
If I use the same html_block and add it to the default page layout it appears as expected on the collection page (as well as all other default pages).
If I change to the bootstrap5 template and apply the custom layout using the same block it works.