You can check the Authorization Scope of indvidual pipelines from here:
https://dev.azure.com/%7Borg%7D/%7Bproject%7D/_apis/build/definitions/%7Bbuild_id%7D?api-version=7.19

I was having issues with just a small number of pipelines that could not access shared resources. In my case it was pipline trying to pull a file fomr a repo in a central repo.

look for the "jobAuthorizationScope" and make sure it is set to "projectCollection" if it is set to "project" then the only way to change it at this point is to use the Azure DevOps rest api.

I have spoken to Microsoft Support and so far I have not had a definative answer as to why this setting was unexpected for a handfull of pipelines.