Here is an oddity I just noticed, but whether it relates to the missing ship completion drop downs or not I can't tell.
A planet completed the social improvement "basic factory" when the turns to complete a ship changes from one week to 0 weeks, the ship gets completed (as might be expected) but the drop down for the ship does not appear. So far, I have only observed this once that I know of. But then, this is the first time I have been watching closely.
Perhaps it is the result of the ship being completed more quickly. Perhaps it is because one drop down for that planet has already been queued. Perhaps it is something else entirely.
Another thing I noticed, the ship time to completion (except as observed above) will sit at 0 weeks for two turns.
Watching social completions is a bit more difficult, so I am not sure, but I am getting the impression that social completions occur a week early (no "0 weeks to completions"). I am still watching this and will post my observations if it differs from this.