I have the Same problem

Played Jund sac with jeganther in the sideboard which whenever I tried to side in (removing bolas's citadel from the main deck) would cause this issue.

Sideboarding worked with my lurus deck

Same here, Bo3 unplayable, playing Keruga deck, is this related, is there a workaround?

Same. Cant sideboard while Kaheera is my companion. Im not sure if companion is the issue but thats the deck that encountered this issue. After 5 games of not being able to sideboard, i reinstalled Arena. I was able to sideboard for one game, then the problem returned. Pls fix

Same issue for me as well yesterday. Tried uninstall/reinstall to ensure there wasn't something strange going on. This happened for me with both Yorian and Kergua decks. I use the untapped plugin which showed my sideboarding correctly.

Then a game with Bant Yoiran, I had sided in Dovin's Veto and didn't see it in the list with a Tamio activation. Sadness.

I have the same issue with a Umori, the Collector deck. Maybe it is generally related to having a companion?

There is also an issue when having played, for example Traditional Standard ranked, and changing the deck after. If the deck is not double-clicked before hitting the "Play" button again, the updates are not live.

Is this related to Keruga as your companion, and you are trying to add CMC < 3 cards? I see the same bug.

same thing is happening to me, i thought I was going crazy but then realised its a bug

I am experiencing the same issue. I will side cards out and still draw them G2 and when boarding again for G3, it shows that I submitted my G1 configuration of the deck.