Firebase security rules: some string fields not accessible with resource.data

Question

I want to set up my security rules and want to make certain collections only accessible to the document owner. For that I want to access e.g. for the projects collection the field projectOwner with resource.data.projectOwner. Unfortunately this doesn't work for this and some other fields. (I tried to test it with resource.data.field and fields I knew were not empty) and only with some fields I was able to see documents.

Originally I tried to see if the project owner is in the user teamID (String List):

allow read: if request.auth != null && resource.data.projectOwner in getTeamID(request.auth.uid); (I have the firebase authentication setup)

I first tested the function getTeamID but this works as expected. Then I tried to test projects fields: allow read: if request.auth != null && resource.data.projectOwner != null; but that only works sometimes, although the values are set correctly.

My default is: match /{document=**} { allow read, write: if false; }