Okay, nevermind, I just figured it out.
Because I haven’t worked on this website for a while, I completely forgot that the single custom template was assigned to a specific category.
After searching, I found that the right template was still intact. However, because of the ACF update, the custom fields I had set up in admin disappeared. When I recreated the field groups, I luckily assigned the right names to the fields that showed up, and unfortunately assigned different field names that didn’t show up.
Hope this would help anyone that encountered this type of problem.
