This has happened three different times for this game. I have a physical copy, and all three instances the game has froze differently.
The first two times I was visiting a friend's town, and we both had been fishing at the island. We had been playing for a couple of hours, had gotten TONS of sharks and beetles on the island, and the first time my game just black screened.
I was fairly upset, but I thought it was just a rare occurance and restarted my system and visited him again, only for my system this time to just graphically freeze, and I was teleported back to my town and so was he, losing our items from the island. So I looked online on help forums and thought it may be my system or sd card.
I went and purchased a brand new 3DS XL and transferred all of my stuff to a new SD card today. My friend decided to come and visit my town this time, and we went to the island. We had been fishing and catching bugs for a few hours, and again I had a ton of sharks and beetles, and I decided to release a Sea Bass. The moment I released it, my game froze on the menu screen. I could not get off of the screen, yet I could see my friend still running around and the water animation and the little animation behind the menu screen was still moving and I could not click on anything in the menu. I tried everything, pressing many combinations of buttons and trying to close and reopen my 3DS to no avail. I waited for almost an hour before giving up and turning off my console, once again losing thousands of bells worth of items.
Can someone please help me?