After a split, the source discussion is shown as read even when there are new posts

[ekumanov]

Bug Report

Current Behavior
When the split function is used to move posts to another discussion, then the source discussion is shown as read even when there are new posts in it.

Steps to Reproduce

1. Split a few posts from a discussion
2. Make a new post in the original discussion with a different user
3. With the original user that split the discussion, open the discussion list.
4. The discussion is bumped on top, however it appears as read (no bold title)

Expected Behavior
The discussion should be marked as unread if there are new posts

Additional Context
According to the user matteocontrini who first reported the issue on Discuss, the issue will resolve itself once the number of new posts exceeds the number of moved posts. Also, he said the issue is observable only by the user who performed the split, I cannot confirm this, hasn't tested with many different users.