When a new branch is created, some work needs to be done to specify that the previous branch is no longer equivalent to HEAD.
Edit /var/portbuild/conf/server.conf with the following changes:
Add new-branch to SRC_BRANCHES.
For what was previously head, change SRC_BRANCH_branch_SUBDIR to releng/branch.0 (literal zero).
Add SRC_BRANCH_new-branch_SUBDIR =head.
Run /var/portbuild/updatesnap manually.
Note: Again, as portbuild:
For each branch that will be supported, do the following:
Kick-start the build for the branch with:
build create arch branch