Issue ordering is screwed up automatically in issue boards

Summary

Given that we have a Product Manager
and an issue board with scoped labels as columns
and the PM made every column on an issue board priority ordered
when the PM assigns a scoped label to an issue
then it might end up "anywhere" in otherwise priority ordered column of the issue board.

As the developers would like to pick the top item from the workflowready for development column, and the column's ordering changes without any user intent, it's really hard to keep the developers informed of what is expected to be picked.

Steps to reproduce

Given that we have a Product Manager
and an issue board with scoped labels as columns
and the PM made every column on an issue board priority ordered
when the PM assigns a scoped label to an issue
then it might end up "anywhere" in otherwise priority ordered column of the issue board.

Example Project

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What is the current bug behavior?

The newly labeled issue might end up being the first one in an otherwise priority ordered list.

What is the expected correct behavior?

The newly labeled issue should be the last item in the list.

Relevant logs and/or screenshots

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Output of checks

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Results of GitLab environment info

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`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

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sudo gitlab-rake gitlab:check SANITIZE=true)
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sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)