Error shown when trying to set assignees

**Describe the bug**
Setting the assignees for this particular issue did not update the assignees. I got an error message instead. But after going home (back), I saw that the assignees were set for the issue.

**To Reproduce**
Steps to reproduce the behavior:

1. Respond to the issue by changing severity, rejecting it, and adding a screenshot.
2. Click on the issue from the 'Issues responded' table.
3. Try to set assignees (I tried my own handle).
4. Error displayed.

**Expected behavior**

The user should have been assigned.

**Screenshots**
Screenshot 1:

Screenshot 2:

Screenshot 3 (Home):

Screenshot: 4

**Desktop (please complete the following information):**

- OS: macOS Intel
- Browser: Safari
- Version: I guess the latest? I'm using Monterey

Start by reproducing the flow from the Issues responded table: change severity, reject the issue, add a screenshot, open it, and set an assignee. Inspect the issue-detail assignee update path and verify whether the displayed error occurs despite the assignment succeeding. Done means assigning a user shows no error and the assignee remains set after navigating home and back.

Written by the indexing model from the issue text.