I can't fully pin this down, but I and a couple of others I've discussed this with are seeing the following behavior.
With this example: a citation [cite:@couper2014] and another [cite:@kohn2006] ... the first citation is highlighted, and the second (and any subsequent) is not. In some way I can't reproduce, I can then get the fontification to work correctly, but if I restart, the problem reappears. Discussed more here: https://github.com/bdarcus/citar/discussions/430#discussioncomment-1693707 Can anyone else confirm this? Bruce