Custom block being saved as core block

I have setup a custom block (my-block/my-first-block). The block works when adding it to the editor, but after saving and reloading the page I get this message:

Your site doesn’t include support for the “core/my-first-block” block. You can leave this block intact, convert its content to a Custom HTML block, or remove it entirely.

In the database the block is being saved as <!-- wp:my-first-block -->

I’m enqueueing my block script with the enqueue_block_editor_assets hook and I’m registering my block in php on init.

In the JS, I return null in the save function, because I’m using dynamic rendering.

I have found other threads where a similar problem has occurred, but never where it seems the block is being saved as a “core” block. What could cause this?