By setting a Manga directly as Completed which didn't previously have a status of reading, and so no progress number or start date yet, and saving it with all fields populated will not send complete informations to AL.
Checking it out after the saving you will see it Completed but Start Date and Chapter progress are empty.
It sets the completed date, since that's when you finished it, but it can't know when you started it. In some cases, when you're reading a one-shot, it would make sense to have started and finished it in the same day, but we can't make that assumption for everything.
It should set the progress to max, but it won't happen, if the manga doesn't have it's chapter count registered on AniList. If you try this with a manga, that has its chapter count registered, it will work. If you have a concrete example, where that doesn't work, please share.
By setting a Manga directly as Completed which didn't previously have a status of reading, and so no progress number or start date yet, and saving it with all fields populated will not send complete informations to AL.
Checking it out after the saving you will see it Completed but Start Date and Chapter progress are empty.
Version 1.3.2