I am playing Magic Arena on my Android Galaxy Fold 6.
After going to my collection and into my deck, I added four Malicious Eclipse to my main deck.
After Malicious Eclipse was added to my main deck, I when to move it to my side board by dragging and dropping it from the main deck to the side board. When this is done, Malicious Eclipse will not move too my side board.
I dragged other cards from my deck, Stock Up, to the side board and it was successful.
I closed the app and cleared the cache, but the problem persists.