That's strange, as I've been on 12/12... Until the duplicate one was completed... I know what I've seen, I know that I've gotten rewards for both the first and second copy of the day 12 quest, and I know the count went up after completion of both. You can keep saying I don't know what I'm talking about, but I've been closely paying attention to things. Even before the valentines event, I knew something caused the missions to be rerolled, as I saw the rewards change at times (which caused me to lose Koban reward missions until I started doing those before any others, as active mission slots don't get rerolled if that happens). During the first day of the valentines event, that happened and then I had 2/12 on day 1. On day 10, I was at 11/12 and on day 11 I was at 11/12 too, and then I went from 11/12 to 13/12. This makes me believe something prevents it from going to 12/12 or above until the 12th day mission (or later) is completed, and it also explains why people think may it's just a visual bug. Having a sanity check to prevent the event from being completed before it should be possible makes a lot of sense. During that event I ended up at 15/12 at the end of the event. I don't know how it reacts if you don't complete a duplicate for an event quest after it's generated, but I assume it may still count it in the total as the same day flag is used for both and depending on how it's set up serverside it may have a duplicate that counted without the player knowing it existed.
During the Spring event, I didn't get a duplicate so I couldn't test anything related to it, and I ended up being on 12/12 on day 12 and 14/12 on the last day. This morning I had 2 copies of the Lent day 12 event. This morning after completing the first copy and until I completed the second copy, I was on 12/12. If it's just a visual bug, as you claim, then it shouldn't be possible to be at 12/12, but I was. I refreshed and reloaded the missions page a few times until the second one completed, and it stayed at 12/12. It did not go to 13/12 until the second copy was completed. I made a few screenshots of several situations related to this, including the 12/12 state. This 12/12 state does not exist according to you, so I included the screenshot of it. If you think it's just a visual bug where the only thing that happens is that you skip from 11 to 13, then explain how I could have been at 12?