Sometimes CI_BUILD_TAG retains the value of last tagged build in non-tag builds

Summary

Sometime in non-tag builds CI_BUILD_TAG is not empty! (it seems that it has the value of the last tagged build)

Steps to reproduce

I don't know if it always happens. Should check more. But it has happened for me in 2 projects.

Expected behavior

CI_BUILD_TAG should be only set when a tag is built

Actual behavior

It has a value when a regular commit has been made.

Results of GitLab application Check

Seems OK.

Results of GitLab environment info

System information
System:		Ubuntu 16.04
Current User:	git
Using RVM:	no
Ruby Version:	2.3.1p112
Gem Version:	2.6.6
Bundler Version:1.13.6
Rake Version:	10.5.0
Sidekiq Version:4.2.1

GitLab information
Version:	8.14.4
Revision:	3ea6c8d
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		http://gitlab
HTTP Clone URL:	http://gitlab/some-group/some-project.git
SSH Clone URL:	git@gitlab:some-group/some-project.git
Using LDAP:	yes
Using Omniauth:	no

GitLab Shell
Version:	4.0.3
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:		/opt/gitlab/embedded/bin/git