I created a ROA to authorise an AS to originate one of my prefixes, but now another announcement of this prefix, from a different AS, is "Invalid". Why is that?

When you create a ROA to authorise a certain AS to originate one of your prefixes, any other AS that also originates it is automatically marked as unauthorised, i.e. a hijack. So when you are multi-homed and originate your prefixes from several ASs, you have to make sure you have a ROA in place for all of them. This also applies to more specific prefixes that are announced by your customer’s AS.

In short, when you create ROAs, you have to make sure you authorise all of the Autonomous Systems from which your prefixes will be originated.

You can find more information on ROAs, including several examples, on the Route Origin Authorisation (ROA) page.