I attached my Github repo to a Static Web App hosted on Azure. The repo is inside an organisation account, and it previously had Actions disabled. Authorising it for Azure seems to have fixed that: in the organisation settings the Actions got enabled automatically, and a new .yml file appeared under .github/workflows.
However, I the Actions tab did not appear, and trying to visit the actions from within Azure returned 404. This issue is very similar to Actions tab missing, /actions URL 404, /runs work on Github Community (the OP got it fixed manually by Github staff)
It seems the cause of the problem is the Actions being disabled before integrating a repo with Azure. To solve, delete the Static Web App from Azure completely (you may want to back up your environment variables if you have many), go to your organisation settings, and enable all Actions. Then you can re-add Azure.