The only thing I can think of without looking too deep into this is that the character jumps up onto that cube but the movement component never detects that it has landed, so the character remains in a falling state and slides off the cube (Some internal Character Movement Component logic).
If you are using the Character Movement Component or something similar there should be some functions like “Is Falling” that’ll return a bool. Have those printed out each tick and see if they’re what you expect when the character is on the cube.