Bug: KHM card "Poison the Cup" does not scry
The card "Poison the Cup" should scry 2 after resolving, if it was foretold.
It destroys the creature, but does not scry afterwards.

I'm not able to reproduce this. Do you have a screenshot of the board (maybe there was a card preventing you from scrying or some other card has a bugged interaction with foretell)?

I also just had this issue in a sealed game. No relevant effects that should be preventing me from scrying, definitely cast from exile.
I'll remove the "not confirmed" from this since multiple users have reported the bug (and related #7539) but I'm still not able to reproduce it. Need more details from people experiencing the bug.
"foretellAbility" usage must be checked. It's uses different code in different places. See case 1 and 3 for bugs.
case 1 from AlrundGodOfTheCosmos: checking exists of exile zone (wrong: it will be exists all the time after first usage, e.g. that check doesn't work correct for blinked cards).

case 2 from ForetellCostAbility: checking state values, exists of exile zone and card in that zone (can be correct)

case 3 from ForetoldWatcher: checking nothing (wrong: always true, e.g. watcher marks any cast from exile as foretold)

Also I recommends to look at comment for potential use case (blinked cards problem?):
Check that the card is actually in the exile zone (ex: Oblivion Ring exiles it after it was Foretold, etc)
Related to #7677 .
Just experienced this again today
Foretold on T13.M1, cast T16.DA
Here's the screenshot.
I think that Poison The Cup was cast on opponents turn after foretelling it on my turn (just before that turn). Not certain about that though.
Wanted to add more info to the issue. Few weeks ago my friend foretold [[Haunting Voyage]] and later tried to cast it, but only got two creatures instead. I tried really hard to replicate this issue, but alas, still couldn't. I think this may be a general problem with cards who have effects "If this card was foretold", but I have no idea how to replicate this
Haunting Voyage - (Gatherer) (Scryfall) (EDHREC)
{4}{B}{B} Sorcery Choose a creature type. Return up to two creature cards of that type from your graveyard to the battlefield. If this spell was foretold, return all creature cards of that type from your graveyard to the battlefield instead. Foretell {5}{B}{B} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)