In a multi-branch pipeline, if you commit to the master branch and print out the values of the environments variables you will see:

BRANCH_NAME = master
GIT_BRANCH = master

If you trigger a build via committing a tag to the same repository and print out the variables (where the tag name is 4.1.0):

BRANCH_NAME = 4.1.0
GIT_BRANCH = 4.1.0
TAG_NAME = 4.1.0

I believe the correct values for the tagged build should be:

BRANCH_NAME = master
GIT_BRANCH = master
TAG_NAME = 4.1.0