Thanks, Armin! Option 2 from the first link worked for me.
Specifically, this approach:
Do not select both the parent AND subcategory. When editing the post, choose only the child category (subcategory) you want it to be listed under.
Note:
The other method doesn’t seem to work—I can’t set one of the categories as Primary. That would feel like the most logical way to me. Is this something you could consider for a future update, or is there a setting I need to adjust myself?
Specifically, this approach:
Alternatively, you can select both the Parent AND Child category but you will need to make the child category marked as primary.