Auto Assign Bot Randomly fails to assign a reviewer or two

[osquitoOfTheNorth]

Describe the bug
A clear and concise description of what the bug is.

Sometimes with some pull requests, auto-assign will assign two reviewers when three are requested. We also maintain two groups.

To Reproduce
Steps to reproduce the behavior:
1.Add the auto-assign GitHub app to a repo.
2.Submit a PR against the repo.
3.Sometimes auto-assign will assign two reviewers, sometimes it will fail to assign any at all.

Expected behavior
Assignment of all 3 reviewers should always happen consistently.

Additional context
A sample .yml file that closely mirrors our github repo setup.

addReviewers: true

numberOfReviewers: 1

useReviewGroups: true
reviewGroups:
  groupA:
    - user1
    - user2
  groupB:
    - user3
    - user4
    - user5
  groupC:
    - user6
    - user7
    - user8
    - user9

[karlb]

We use it to assign a single reviewer, but sometimes (although rarely) no one gets assigned. Maybe that is caused by the same underlying issue.