MyEsri isn't working right now, so I will have to make a bug report later. But I figured I could at least make a PSA in case others run into this same issue.
My Power Automate flow expects that "attachments" always exists (because it should). However, I have noticed some flows were self-terminating. This is an intentional feature which I added in case people don't read instructions.

The problem was that all the terminations I was seeing made no sense.
When I inspected the JSON of the failures, they were all missing the "attachments" component:

After some quick troubleshooting, the issue appears to be with file names. The user was adding a date to the file name, and the comma in the date was causing the issue. As soon as the comma was removed, things worked again:

If you are randomly missing this section in your JSON and are also allowing users to name files themselves, check for special characters (e.g., a comma) in the attachment file name.