I just started a new Jenkins, created a repo in GerritHub and set it up. When Jenkins builds the open changes, all the environment variables are being created and exposed accordingly. However, in a build of the branch such as master, none of these variables are set (only PROJECT, API_URL and CREDENTIALS_ID).
I suppose this is what causes the gerritReview step to fail when trying to post comments on a merged change. (When using plain Pipeline with Gerrit Trigger, the gerritReview step works fine on post-submit changes).