Tip: Dear Diary: Day 13, I seen a man completely erased from existence... like from the sky came some kind of hammer and down he fell. B-but he just disappeared! I must discover what this strange power is...
I've realized that you don't have to jump at an angle to clip through, straight up still works:
if mario is $0008 to $0009 units (in pixels) to the right from the block, you can clip through that bottom-right corner.
What happened is that mario's head collision point (this interacts with MarioBelow is not triggering the one_way_down_right [broken], rather the block to the right of it, hence a collision point cannot be on two blocks at the same time, it only picks one. When going upwards far enough, the only way to check this collision is via MarioSide and Headinside as that detects the leftside of the player.