As per this official document of IAM cloud build has permissions for overall build not for triggers. It means we have permission for build-level not for trigger level. Cloud Build Editor has Full control of Cloud Build resources that is the reason you are getting full access or no access, I have tried to reproduce the same scenario but am facing the same issues.

My suggestion is to have separate dev, UAT, and PROD projects and so that you can provide the permissions as per your requirement.

I have checked for feature requests or bugs on it but it seems any issues are not created till now so if you are not satisfied you can create a new Issue tracker thread describing your issue. If you are using paid support you can create an issue.