Mystery box
Just popped my mystery box. Then I tried to use incense, and the game told me something like "you're already using an item of this type". Okay, guess I won't double up on that.
Also, just after the first Meltan popped, I got a Gulpin. As of this message, I'm 15 minutes in on the Mystery box, and only have 4 Meltan - I hope it didn't glitch one of the 10 I am supposed to get into a Gulpin. We'll see.
Answers
Out of curiosity, did either of the following happen while you had the effects of the box active?
• You momentarily lost your GPS signal. This could potentially mess up your spawns or cause you to miss one.
• Weather changed. What you experienced is something that I've read about before, and its happened to a few people in my local community as well. Some believe that if you have the box active when the weather changes, it could cause you to receive a weather-boosted spawn that isn't a Meltan.
Per my journal, the first Meltan was caught 9:55 AM. Gulpin was caught 9:57 AM. Next Meltan caught at 10:01 AM. So it seems unlikely the weather change did it, unless it was the spawn just before the weather switch.
I seriously doubt that I lost GPS, as I was at home on Wi-Fi the whole time. The Gulpin spawn was weird in that there aren't any spawn points near my house, and the Gulpin appeared right at my avatar's feet, just like the Meltans did.
Not the end of the world, I'll be spawning 10 of these about once a week going forward, so in the grand scheme of things, no biggie.
Mr-ex777: How dare those #@&!# #@*!# at Niantic murder your Meltan. You've been raped, this game is $#@!$#, and it will die a horrible death sooner rather than later.
Normal person: Might be prudent to not open your Mystery box right around the turn of the hour, to avoid these problems, despite the journal evidence of the exact times of catching (or spawning, whatever it records).
Based on your catch times, I'm now hesitant to think that weather played a role. If you had caught the Gulpin at 10:01 and the weather was cloudy, then I would be inclined to think "yep, weather did it." But since the Gulpin was caught at 9:57, that makes me think that a weather change wasn't the deciding factor.
This could also very well just be a bug where, sometimes, the mystery box mistakenly spawns a random Mon in place of a Meltan. If it is something we can identify and potentially control, then I hope others share any similar experiences they run into so we can try to identify any commonalities.