Account synced blueprint books

[Tertius]

A warning would be fine, since Factorio isn't able to merge blueprints from the cloud with local blueprints created while not cloud syncing.

As far as I remember, Factorio will destroy cloud-stored blueprints, if it was running at least once with a game running and without blueprint cloud sync active, then cloud sync enabled in the options, then Factorio exited. In this moment, at exit, Factorio will upload the local blueprint to the cloud, because cloud sync is enabled, and it will upload an empty blueprint library if the game was just installed and once running with some game, so an empty local library was created.

What you need to do is enabling cloud sync immediately at the first start of Factorio on a new install, and not start a game (so Factorio doesn't create a local blueprint library) but instead exit Factorio immediately. This way no local blueprint is uploaded, because there is none, so the cloud stored blueprint library is not destroyed. At the next start of Factorio, it detects blueprint cloud storage is active, so it downloads the current blueprint library, and you're up and running with the current blueprint library from the cloud.

If you want to make sure you don't destroy a cloud stored blueprint library after you activated blueprint cloud storage, you need to delete the local blueprint library before you exit the running Factorio.
Delete these files:
%appdata%\Factorio\blueprint-storage.dat
"C:\Program Files (x86)\Steam\userdata\<your steam user id>\427520\remote\blueprint-storage.dat.zstd"
Then exit Factorio.
If you delete these, Factorio isn't able to upload at exit, so it isn't able to overwrite the cloud data. Please be careful, this is from my memory from some blueprint restore action 2 months ago and might not be 100% accurate info.

What is accurate is that the file blueprint-storage.dat.zstd is the file that's actually being uploaded/downloaded to/from the cloud. I guess, on startup, Factorio downloads blueprint-storage.dat.zstd and extracts it to %appdata%\Factorio\blueprint-storage.dat, where Factorio is using it. On shutdown, Factorio compresses %appdata%\Factorio\blueprint-storage.dat to blueprint-storage.dat.zstd and uploads this.

What Factorio probably needs to do to fix this issue is to immediately download and extract the current blueprint-storage.dat.zstd if the user activates cloud sync in the options, overwriting any local blueprint library. Or to inhibit the upload after program exit, so downloading and overwriting of the local storage takes place the next startup.