I've had that. In fact I had it this morning (yesterday I was 3 out of 8 deathclaw attacks and this morning it was back to zero).
I've also had it when I was at (something like) 5 out of 7 mole attacks which was suddenly at zero and a very annoying one when I was something like 215 out of 270 outfits. Back to zero again!
I've also got the well known "ghost in rooms" bug where several of my fully upgraded, 3 x span rooms apparently have corpses in them. When they don't. The dwellers there are all unhappy and the only way to fix it is to destroy the room and start again. I've "fixed" several rooms this way but have many more to go. What a waste of caps...
I also *think* I've seen a bug where a dweller returns with a really interesting item. You collect them and the item doesn't seem to be in my inventory. Weird.
I really hope that a new version with some bugfixes comes out soon. There's quite a few UI issues that could be tweaked as well....
I was thinking that there might be a time limit as it always seems to be the ones that take forever to complete that zero out - maybe just a coincidence but I've never had an easy one do it.
I'm having similar issues. I had two long-term achieves reset to zero today. One was selling 107 weapons/outfits (I was 2/3 of the way through) for 1440 caps, the other was "collect 20,000 caps in the wasteland" for a Mr. Handy (I was 18,000 through), which is what basically made me rage...
I noticed this happened to me when I exited my vault, stayed in the Main Menu for a little while, then reentered the vault. I then sold an item I looted from a raider, and the message popped up saying "1/107 weapons/outfits sold," to which I replied "excuse me?" That's when I found out that the Mr. Handy achieve had also reset...then the rage... :brokencomputer:
At this point, I feel like I deserve a Mr. Handy! But c'est la vie, I suppose...It's only a game, after all... :sadvaultboy:
I have an achievement of selling 1,620 weapons for a lunchbox, and it has reset to zero on me TWICE so far.
About a week ago it reset to zero when I had about 500 sold items. This afternoon I see that it reset to zero on me again after I had gotten back up to about 1,200 sold items. Between the two resets, I would have had this completed already.
Gah, this is such BS! But then again, the lunchbox's "rare" card is probably 500 caps so I'm not missing much I guess.