This was observed on srwiki, but probably happens with all languages with variants, thus title of the ticket reflects that.
Here is gif that should help you understand what happens:
In the gif you can notice following behavior:
- Ћир./Lat. is selected by default in language variant picker
- Menu is opened and I choose Cyrillic script (with Ћирилица label)
- Page is reloaded with Cyrillic script chosen and we see the bug: bosanski (Bosnian), hrvatski (Croatian) and српски (Serbian) are shown in gray links, marking the translation missing in those languages. This really looks wrong, as I'm on srwiki actually.
- After reload, Ћирилица (Cyrillic script) is selected. I go ahead and choose Latinica (Latin script).
- Page reloads, showing that both non-default language variants have the same problem.
- Now that Latinica (Latin script) is selected, I go ahead and once again choose the default option (which is Ћир./Lat.), where we again see that default version doesn't have this wrong gray link problem.
Clicking on the gray link for Serbian, typing the title (different from original one), leads to Content Translation view, where I can translate the page without any notice. We can probably end up with source and target languages in other ways, and I'm not sure if choice not to warn is deliberate and use case considered valid.
You can, of course, publish the page, creating the (near) duplicate.
Template adaptation didn't work, which was surprising to me. Showing no warning makes me think this is valid use case, but trying to adapt the template and failing to do so is interesting.
Also, translations in progress shows "An hour ago" which is certainly incorrect, but this is more likely deliberate and though about.