Collectible Card Game Headquarters

Describe the Bug:
If you Fork a Might of Old Krosa that you cast during your main phase, the copy's target gets +4/+4.

Which card behaved improperly?
Might of Old Krosa.

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Dev e9d990e.

What exactly should be the correct behavior/interaction?
The copy wasn't cast, so its target should only get +2/+2.

706.10. To copy a spell, activated ability, or triggered ability means to put a copy of it onto the stack; a copy of a spell isn't cast and a copy of an activated ability isn't activated. [...]

Are any other cards possibly affected by this bug?
The other four cards with this wording are Sulfurous Blast, Haunting Hymn, Careful Consideration, and Return to Dust.

Effects that copy cards when they're not on the stack and then let you cast them result in copies that are cast. Isochron Scepter is a prominent example, but there are plenty of others.

I don't think any cards that let you gain control of a spell on the stack (Aethersnatch, Commandeer - are there any others?) are in Manalink. If any were, these cards would also have to check who originally cast the spell, not just who currently controls it.

So is Fork cast to trigger Abbot of Keral Keep 's ability but the copy that is placed on the stack is not considered to have been “cast” copying Might of Old Krosa right?

It's not clear what you're asking.

If the Fork is cast to trigger Abbot of Keral Keep's prowess ability, it's irrelevant.

If the top card of your library when the Abbot enters the bf is Fork, and you play it from exile, that's casting the Fork. (Rule 701.12b: "To play a card means to play that card as a land or to cast that card as a spell, whichever is appropriate.") It doesn't matter whether the Fork was cast, though; it matters whether the Might of Old Krosa was - and for that, it doesn't matter where the copy effect comes from.

If the top card of your library when the Abbot enters the bf is Might of Old Krosa, and you play it from exile, that's casting it. But then I don't see why you mentioned Fork.