Editing the Request participant field results in "Could not find usernames" error

Summary

Adding request participants to the ticket will result in errors.

Steps to Reproduce

1. Create an issue
2. Add a Request participant and confirm.
3. Edit the "Request participant" and try saving again.

Expected Results

The field is successfully saved.

Actual Results

The following error appears:
Could not find usernames: $user.name

Notes

(Optional - If Necessary)

Workaround

Clearing the Request participant and adding everyone at once seems to work.