The best tool for debugging is the Nuxt DevTools integration with Nuxt Robots.
This will show you the current robot rules and your robots.txt file.
You can enable the debug option which will give you more granular output.
This is enabled by default in development mode.
When submitting an issue, it's important to provide as much information as possible.
The easiest way to do this is to create a minimal reproduction using the Stackblitz playgrounds: