Standard right-click behavior in AOC/HD has been, that right-clicked melee units will head towards their target, but will attack other enemy units, if they are nearer. This causes units to spread between enemy units and allows right-click to be used in group battles.
Right-click can be considered both as “attack this target” and “attack this target group” (I can’t right-click on whole group, so it is good that I can right-click on single enemy unit in group, but my units will still attack whole group). It is logical for group of right-clicked units to spread between enemy units, because they all can’t fit around one enemy unit anyway.
I tested this both in AOC and HD and in both right-clicked melee units will behave like in second part of my video.
“You command the whole group to attack a single unit, it is expected that the group must follow your command until that unit die.” - I don’t know, where you took this from. This has not been behavior in AOE 2 for 20 years. Maybe you want the previous behavior to be changed, but this change needs to be seriously discussed in community, because is a very big fundamental change for one of the core parts of gameplay (right-click in combat). Also I have to add, that I am heavily against changing previous right-click behavior, because I am used to this and would totally hate being forced to start patrolling like so many other people.
There are significant differences between right-click and patrol mechanics and I prefer right-clicking, because this gives me more control. I can direct my units towards enemy units I want them to fight instead of just fighting everyone on the path like with patrol. Also I like that right-clicked units will just stand where they are after combat instead of moving to patrol/attack-move destination. Right-clicked units also ignore buildings, which is a plus. Right-click is easier to do than patrol/attack-move (single right-click vs keypress+left-click).
But I don’t want this discussion to be about patrol vs right-click. People have different preferences, this is normal. I just want, that one of the core parts of gameplay is left working like it has worked for 20 years. It seems, that this has also been the intent of developers, because right-clicked units still tend to spread between enemy units. Just in case of groups they might get confusing and keep on running/idling instead of finding a target. This appears to be some kind of a pathfinding problem, where they don’t understand that they should switch target.