Form Submission Showing in SQL, Not in ODK Aggregate Web App or Downloaded Data

1. What is the problem? Be very detailed.
Hi all - odd issue that I'm wondering if anyone else has seen before. I tap directly into the SQL database ODK Aggregate runs on top of (in a read-only manner) to pull data out. However, I noticed today that for one form (with a hundred or so submissions), one submission appears only if I query the SQL table directly, and does not show up in ODK Aggregate itself on the web interface, nor does it show up in the downloaded CSV or Excel file if I export the form submissions out of ODK Aggregate. This is only an issue for one submission (to the best of my knowledge) on this specific form. Has anyone seen this before or have any idea what could cause this? As far as I can tell, the only difference between this submission and others is that the _MARKED_AS_COMPLETE_DATE value for the submission that only shows up in SQL is null, whereas it is set for all of the others.

2. What app or server are you using and on what device and operating system? Include version numbers.
ODK Aggregate running on Ubuntu Server 18.x, postgresql