It seems that this JCasC related change broke Credentials selection in the UI (Manage Jenkins):
Although Credentials are present for the given Jenkins instance, the respective dropdown list ist empty. Other Plugins can select the Credentials just fine.
Setting the Credentials via JCasC works as expected but once you submit the "Manage Jenkins" page (with unrelated changes) the Credentials setting is reset to none. This is therefore a rather critical bug!