In /search, you can filter by whoch user has posted the post. This input only accepts 1 user, but it looks like multiple users can be entered, because of the box with the ×, and of the reason below.
The possible reason for the confusion may be due to the input to edit tags, that can accept multiple tags, but also looks very similar:
Is it possible for the username input to be changed so that it is clear that only 1 user is possible to be added?