I had my creature attack with 6 power and trample. My opponent had assigned a 4 toughness no ability creature to block it. So 2 damage should roll over. BUT the game forced me to assign all 6 damage to the blocking creature. (ie no damage hit the opponent)

In the same game. I have a different creature 6 power, trample and my opponent assigned a 5 toughness creature with no relevant abilities to block. The game forced me to assign all damage to blocking creature although 1 damage should've hit my opponent directly.

My wife has also encountered the same issue in a separate game. It's possible that it has to deal with the commander because we both had the experience when the commander "Lightning, army of one" is on the battlefield

I've never had this issue with my green decks that rely on trample.