I ran into this, and got it fixed thanks to people in the MTGA Eternal Discord.

When you look in your sideboard, you'll see the companion both in the sideboard and in the companion slot. Remove it from the sideboard (it'll show 14 cards), and then it'll work properly :)

bumping this. just experienced in bo3 timeless ranked game. 9-13-2025 ~ 10:37 P.M. EPT

Please fix this issue, I keep getting it when I have Yorion.

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Having this problem with a Yorion companion deck. Sometimes I get the whole SB, but most of the time I only get BO1 SB

I really can't understand how this hasn't been solved for over a year now.

Been having this issue with a Lurrus Historic deck. Only 6 sideboard cards available out of 14 thanks to the companion.

Been having this issue for months, sometimes not even having access to 7 cards in the sideboard, not in any order (so no chance of playing Karn) and now I when I try to queue into a Bo3 direct challenge it says my deck is illegal.

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Other Sideboard cards were missing

Bo3, deck registered correctly, only seven cards showing. Today was not the first time that this has happened.

Same here, Yorion deck playing "pioneer". I have lost at least three matches because I can't access key cards in my SB.

only 7 of my sideboard cards show up during best of 3. Please fix. I appreciate everything you all do!

Same here, also playing a Yorion deck

Same here.
I've just played a Bo3 Pioneer match, and it just showed me 7 cards instead of 14 (I was playing Yorion).
To be accurate, it showed just 3 cards which I have in multiple copies in my sideboard.
It's been at least 2 weeks since I've faced this bug, and that's why I was not playing Pioneer anymore.
I thought it was resolved at this point... But yet again, same game mode, same bug.

Launch day for new set and can’t even play game
Lol

Put mascot exhibition in sideboard to fetch with divide by zero for imprint…
Half my key cards are in sideboard..

When I mastermind Karen or learn it only gives me access to 7 cards..

Sideboard should be 25 - 60 extra cards. Instead we are given a measly 15…

Which now we cannot even access.

Lol

Already tanked rank due to 250 hellgolifegainernetdeckbabies

Now basically have to conced match if I can’t fetch lesson

What a joke.

This has been an on again off again issue with sideboard. It’s a recurring bug since day 1

Maybe spend less money printing 12\12 Dino’s that cost 1 mana can’t be blocked and have haste ward infect wither cascade add 9 mana vigilance can’t be countered bs

And more time fixing your game in ways that don’t promote brain dead gameplay

Bo1 doesn’t need sideboard and shouldn’t be a ranked format
Take alchemy out of historic or give us a proper eternal format.
Pioneer is a joke when we can’t play half the cards we own

Still having this issue in BO3 Pioneer. I've only got 7 cards in my sideboard for games 2 and 3, which makes certain ranked matchups nearly impossible to have a fair chance.