Lost submission invisible even after being sent to the server multiple times

We are currently collecting data and our participant's information is split into several forms.
For one participant, the first form is missing, so we have asked our data collector to send the form again.
With screenshots from ODK Collect as proof, we know they have sent it again but I can't find that submission on the data extracted from Briefcase, nor anything matching the interview or submission date on the Aggregate server.

All the other forms have been sent successfully, it's just that one that is missing.
Where could I look for it? I have tried looking on the Aggregate server, the csv files extracted from Briefcase, but can't find it.
Is there a possibility that the form is marked as sent on the tablet while it's not?
There are no media attachments on that form, and we use encryption.

I do not have physical access to the tablet used for collection, but if it's the only way to extract the data, I might get someone on the field to do it.

by other forms you mean completely different forms or just different finalized instances of the same form?

I mean both. Other different forms for that participant, and different instances of that first form for other participants.