Dw about it, tasks do appear and disappear, it is kind of annoying because i was also doing the ones on vast wars. They will come back at some point.
I think it's common, i see players doing tasks of games but when i go to the games they have done i don't have the tasks and misty says they do disappear and appear for a reason. So i don't think it's a bug at all.
I already build a gym but still rejected 
Update: the tasks are back now. Do it while it is still there. Goodluck!
When your tasks get approved, did you confirm your email for this game?
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I've played "Vast Wars" since I joined gamehag four days ago and I've completed 2 of the tasks. Now when I want to report my completion of the 3rd task, all of the links have been removed. Is it common that links disappear on the game pages from time to time? I've already sent Misty a ticket about it, but I also want to ask the community if you've encountered something similar before.