Can't sideboard bug

Reproduction steps:
Playing Bo3 Timeless Ranked
After game one, I move cards in and out of my sideboard.
I click the Done button, but it does nothing.
I wait 90 seconds for the timer to finish.
When the game starts, I mulligan a few times, and see cards that I sideboarded out.
No evidence during play of the cards I sideboarded in.
In other words, my sideboard actions were not saved.
(Curiously, Unplugged shows my deck contents with the sideboard changes.)

Playing on PC with MTG Arena app.

Update: I have deleted my PC app and I am now playing through Steam. This has made no difference to my problem.

Update: I was able to sideboard normally in Historic Play Bo3 and also Historic Ranked Bo3.

Update: Also checked in Standard Bo3 Ranked. Sideboarding works there. So far, my only problem is with Timeless Bo3 Ranked.

Update: I have played a different Boros Energy deck in Timeless Ranked, and I was able to sideboard. I have identified some cards in the Mardu deck that are not in the Boros deck. I have removed these cards one by one in the Mardu deck to test if they are the issue. Bojuka Bog and Cthonian Nightmare have been tested, and I still cannot sideboard without those cards.

Final Update: I was trying to sideboard in Leyline of the Void, which has two mana symbols and does not work with my companion, Lurrus. Arena used to give a warning about adding a sideboard card that doesn't work with a companion, but now there is no warning, and the player simply can't sideboard. I have a workaround to not include any cards with more than one mana symbol in my sideboard.