Trample vs. Creature with "Protection From" a color, Unable to assign post-toughness trample damage.

I was just in a game in which my opponent gave one of their creatures proection from blue. All my creatures had trample. When the trample damage "assign damage" popup happened between my 8/8 with trample (that was blue), and their 10/5 (with "protection from") I was unable to only select up to the expected 5 damage that "would be" necessary to kill the 10/5. The interface intstead forced me to assign all 8 damage TO the creature with proection.

I am going off of MTG official ruling 702.18b - beneath which an example is given:
"Example: A 6/6 green creature with trample is blocked by a 2/2 creature with protection from green. The attacking creature’s controller must assign at least 2 damage to the blocker, even though that damage will be prevented by the blocker’s protection ability. The attacking creature’s controller can divide the rest of the damage as he or she chooses between the blocking creature and the defending player."

The interface seems bugged to not "apply" any damage to the creature with protection. Attaching game log in case it's helpful.