Hey @carolynvs,
Yup, this is expected though I understand not what you were hoping from the feature. You can see more discussion about this, as well as the proposed docs update (and feature request not mentioned there) that came out of that conversation: Docs say Deploy Previews say preview will show result of merge, but it does not - #6 by jen
The TL;DR: for Deploy Previews is that we build the commits you push- there’s no rebasing or post-push updating of the preview based on later changes to the repo.
To get a Deploy Preview with commit B’s changes, you would have to push a new commit to commit A’s PR that includes commit B’s changes. Please let us know if that helps or if there are followup questions!