After playing a best of 3 match with a constructed deck, any changes made in sideboarding are sometimes applied to the decklist permanently.

Steps to Replicate:
1. Log into Arena, and choose a deck to play. Review it to make sure you know it's maindeck and sideboard.
2. Enter into a Best of Three match with the deck. Make sure to sideboard between games such that your deck contents for games two and three are different than the normal list.
3. Finish the match.
4. Return to the deck editor, and select your deck. You'll be brought to a strange interface, where your collection will be replaced with your maindeck and sideboard, and your sideboard section will be empty. Your sideboarded changes will still be in effect.
5. Select Done. Don't select anything else
6. Select your deck again. Now, the maindeck, sideboard, and collection will look normal, however your sideboard changes will still be in effect.

Recording was to big for the native upload (contained an entire match), so here's a drive link: https://drive.google.com/file/d/1iPc1-DWKR3GNRWAdlIqx2oQmszCWTqmf/view?usp=sharing