Quick Action Adds Multiple Labels to Issue If Middle Words Overlap with Existing Label

Summary

When using the /label quick action and adding a multi-word label, if the middle word(s) (i.e. not the first or last word) overlaps with an existing label, it will add all labels that match, not just the specified one.

See example below.

Steps to reproduce

Create labels: A B C and B
Create new issue and add label A B C using quick action. i.e. /label ~"A B C"
Once created, issue will have labels A B C and B

Example Project

Used this template, which only has the GitLabber Gold Request label: https://gitlab.com/gitlab-com/support/dotcom/dotcom-internal/blob/master/.gitlab/issue_templates/gitlabber_gold_request.md

Example issue had Gold label added in addition to what's in the template: https://gitlab.com/gitlab-com/support/dotcom/dotcom-internal/issues/85

Note: I already removed, the Gold label, but I never explicitly added it.

Issues 9-11 in our test project (internal) show that this does not happen if it matches the first or last word in the string.

What is the current bug behavior?

Adds multiple labels.

What is the expected correct behavior?

Add only specified label.

Output of checks

This bug happens on GitLab.com, 11.4.4-ee