For the “official” providers – that is, the ones in the hashicorp namespace – the registry synchronizes with the HashiCorp Releases service, rather than with GitHub. GitHub is used only for partner and community providers.
However, I can confirm that right now I see 5.18.0 on the releases service but currently the latest version on the registry is 5.17.0.
I personally cannot explain that discrepancy since I don’t work on the hashicorp/aws provider, but it does seem unexpected indeed. I would suggest opening an issue in the provider’s GitHub repository so that the team which maintains the provider will see it, and then hopefully they can debug whether something went wrong with the release process here which prevented the registry from synchronizing with the releases service.