Was there a recent change to how the Scheduled Build->Advanced form validates and renders the form? I remember that if the tag or commit fields are used it automatically disables the branch field. This specific post (Question about BITRISE_GIT_BRANCH env variable - #2 by jamesradford) even implies that the branch field is not set if the tag field is used to trigger the build.
As of writing, the Branch field remains active and editable when the tag or commit is used. The value remains to be the main branch of the application, and it is used to set the BITRISE_GIT_BRANCH environment variable whereas before this was not set since the branch field is deactivated if one of the other fields is used as the build starting point.