Seems like no one except me knows how to reproduce this bug.
1. Have game open on phone (pixel 7 pro for me)
Play game, halfway during game if you close and re open the app you've done it. You can't sideboard. Even if you open the app on pc the side boarding still doesn't work. you can click them all you want it just doesn't work.
2. Android, then pc
3. no sideboard
4. Sideboarding
6. There is no workaround