I am no expert, but I recall back in 2017 Snapchat was different than it is today. If someone opened the Snapchat app and took a photo with the app (not on the camera on their phone) and posted it, then it wasn't stored on the phone. You wouldn't be able to go into your photos and see the photo you took later becuase it wasn't stored on the device. I am unsure if that is still how this works because I no longer use Snapchat.
My kids use Snapchat and every single time they snap someone they take a photo of themselves or something they are doing or looking at and then send that with the snap.. so I'd say nobody would have phone storage space if every single photo was stored on the phone.
IMO this could be why they couldn't find the photo on the phone.