On mobile web, the non-upvoted comments are not expanded by default, only on Workplace.se. No other site does this, including this Meta. Is this by design or a bug?
There is no user settable option for this.
This isn't specific to mobile, and it's an intentional site-specific setting. See Comments without upvotes are now hidden if a question has more than two answers.
The Workplace gets a lot of comments, and the default visibility settings led to a lot of noise on the page that got in the way of getting to the next answer. So we now auto-collapse comments that aren't upvoted on questions with more than a couple answers, to help focus on the important content: the question and answers.