Gaea's Blessing can target illegal targets.

[Tharkon]

When you cast [[Gaea's Blessing]] you can target one player but then target someone else's graveyard. You can even target cards from multiple different graveyards.

The targeted player will put the targeted cards on top of their owner's library. Then the targeted player will shuffle their own library.
Any cards put on top of a library other than the targeted player's will not be shuffled.
This would all be fine as long as the targeted player owned the targeted cards as per the spell's requirements.
But the current implementation does not have that requirement.

Here's an example log:
6:05 PM, T2.M1: Human casts Gaea's Blessing [894] targeting Computer, Grizzly Bears [836] Grizzly Bears [9bb] Grizzly Bears [c71] from hand
6:05 PM, T2.M1: Computer shuffles three cards into their library (source: Gaea's Blessing [894])
6:05 PM, T2.M1: Computer puts Grizzly Bears [c71] from graveyard to the top of Human's library (source: Gaea's Blessing [894])
6:05 PM, T2.M1: Computer's library is shuffled (source: Gaea's Blessing [894])
6:05 PM, T2.M1: Computer puts Grizzly Bears [9bb] from graveyard to the top of Human's library (source: Gaea's Blessing [894])
6:05 PM, T2.M1: Computer puts Grizzly Bears [836] from graveyard to the top of Human's library (source: Gaea's Blessing [894])
6:05 PM, T2.M1: Human draws a card
6:05 PM, T2.M1: Human puts Gaea's Blessing [894] from stack into their graveyard

The card I draw will always be Grizzly Bears in this case.

[github-actions]

Gaea's Blessing - (Gatherer) (Scryfall) (EDHREC)

{1}{G}
Sorcery
Target player shuffles up to three target cards from their graveyard into their library.
Draw a card.
When this card is put into your graveyard from your library, shuffle your graveyard into your library.

[JayDi85]

I think it can be AI only bug due bad possible targets searching, cause card code has all checks and must works fine for human players:

[Tharkon]

I tried it in a duel between two non-AI players as well, same results.

The bug also affects [[Quandrix Command]], [[Dwell on the Past]], [[Witness the Future]], [[Loaming Shaman]], [[Krosan Reclamation]] and [[Stream of Consciousness]] so it's probably either in the code you posted or in some part of code that was copied between all those spells. I can't find anything wrong with it though.

[github-actions]

Quandrix Command - (Gatherer) (Scryfall) (EDHREC)

{1}{G}{U}
Instant
Choose two —
• Return target creature or planeswalker to its owner's hand.
• Counter target artifact or enchantment spell.
• Put two +1/+1 counters on target creature.
• Target player shuffles up to three target cards from their graveyard into their library.

Dwell on the Past - (Gatherer) (Scryfall) (EDHREC)

{G}
Sorcery
Target player shuffles up to four target cards from their graveyard into their library.

Witness the Future - (Gatherer) (Scryfall) (EDHREC)

{2}{U}
Sorcery
Target player shuffles up to four target cards from their graveyard into their library. You look at the top four cards of your library, then put one of those cards into your hand and the rest on the bottom of your library in a random order.

Loaming Shaman - (Gatherer) (Scryfall) (EDHREC)

{2}{G}
Creature — Centaur Shaman
3/2
When this creature enters, target player shuffles any number of target cards from their graveyard into their library.

Krosan Reclamation - (Gatherer) (Scryfall) (EDHREC)

{1}{G}
Instant
Target player shuffles up to two target cards from their graveyard into their library.
Flashback {1}{G} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Stream of Consciousness - (Gatherer) (Scryfall) (EDHREC)

{1}{U}
Instant — Arcane
Target player shuffles up to four target cards from their graveyard into their library.