Sorry, I forgot to mention that I checked there already, and I have no extra code written.
This is all my functions.php file contains:

EDIT: I discovered the issue myself and am sharing in detail in case anyone else has this issue in the future:
When I disabled comments, I did it by clicking on the “Screen Options” button at the top of WordPress within the individual posts themselves. Once clicked, a list appears and you need to check the box for “Discussions”. Afterwards, you scroll down to the bottom of the page and you should see a new settings section for discussions - check or uncheck the box next to “Allow Comments”.
In my case, I had disabled them comments in this way, and then I suspect, perhaps after an update or something, the post defaulted back to that settings section being hidden, but the comments remained off. So, when I went looking, it appeared as though I hadn’t used that method because the section was missing from the post. But, after I made the discussion settings visible again, and checked the box to allow comments, they appeared on the post once more.
For a video demonstration on how to activate/deactivate comments in this way, you can refer to this X Theme forum post: https://theme.co/apex/forums/topic/how-to-disable-comment-section/#post-23787
The link to the video is in the second line of the second comment.