Moved item can end up deleted on sync about unicorn

[cassidydotdk]

You really should have only 1 or at most a couple of Unicorn configurations.

That aside; Unicorn can handle a move across configurations under other circumstances. I think you're possibly stumbling somwhere, in how you execute the above steps. E.g. not ensuring your serialization datastore is kept in sync properly and/or not executing a full sync when required.

existing item under Project.Website (Helix project layer)
move it to Feature.Something project

Aight. So on the instance you're doing this, the .yml file will be deleted from Project.Website and a new .yml file will appear unter Feature.Something

do a unicorn sync and notice the order in which the projects are synced

So the question here is; how did you move your files to the target environment where you now execute this sync? Did you make sure to clean out your deployment target so that the (on local) deleted .yml file from Project.Website got cleared out?

first, the feature is synced: the item is [U] by unicorn (I think the update includes the path, among other fields)

This would suggest you did not.

after that, the Project.Website is synced: the item is [D] by unicorn, "because it did not exist in the serialization provider"

And this is expected behaviour.