So the backup was made by the server, but the unfortunate thing seems to be that the server didn't receive the changes yesterday...
Our current guess is that it's a combination of the downtime a few days ago and our sync backing-off mechanism. We've urgently released a version to disable to back-off mechanism for now.
I totally understand, and that's what we strive to do as well, just like how we strive to keep our servers up as much as possible. We're investigating and fixing right now!
To gather some more information, did you see a "Are you sure you want to leave this page? The changes you made may not be saved" pop-up when trying to close the tab?