Comment on diff partially broken after updating to 8.11

We recently updated our test server to 8.11.0 (e249da43). I just attempted to test out the new "resolve discussions" feature, and encountered a problem.

Steps to reproduce:

1. Create a merge request with the server at 8.9.5
2. Upgrade the server to 8.11.0
3. Attempt to leave a comment in the diff for the previously created merge request

📝 It is unclear to me whether the upgrade process was partially responsible for this behavior.

Edit: I did not observe this behavior for a merge request created after step 2 was complete. I only observed the broken commenting functionality when commenting on a merge request created prior to the upgrade.

Actual results:

1. When you click the Comment button, it turns gray but does not do anything
2. When you refresh the page:

• The comment appears
• The comment does not have a "resolve" button
• The top of the view shows "1/1 discussions resolved"

Expected results:

1. When you click Comment, the editor goes away and the newly created comment takes its place
2. The comment has a "resolve" button
3. The top of the view shows "0/1 discussions resolved"