Hi @Lunatique,
I believe you are correct. Based on your troubleshooting, it sounds like the updated text in iOS Scriv never made its way to Dropbox.
I hate it when I lose work, and I hate it when others do as well. 
As iOS Scriv does not have the auto-backup capabilities of the desktop version, I have forced myself to adopt a manual approach.
Whenever I wrap up a writing session or have written too many words to lose, I use one of the methods discussed in the link from @kewms upthread to back up my edited project: either zip & export the project to a cloud service, or zip & email it to myself, or make a copy of the project and move it to the “On My Device” section of the Scriv projects screen.
That way I have something to fall back to, if Dropbox syncing fails. Which it rarely does–but like I said, I hate losing work.
I know the above approach will not help you recover your recent lost work, but if you adopt a similar strategy, it will go a long way toward mitigating the risk of this ever happening again to you.
Best,
Jim