The payouts that you see on a post reflect a rolling 3 day average of the price of Steem (SBD doesn't come into play there) - what upvotes actually give you are "rshares" (reward shares) which are tiny tiny little slices of the eventual reward pool payout. The actual value of the rshares you have received through upvotes on a post varies with the market price of Steem. They use a rolling 3 day average so it isn't as volatile but when there is a big drop in price that sustains over several days, yeah, you will see pending payouts crater.